Categories

## Fair coin Problem | AIME I, 1990 | Question 9

Try this beautiful problem from the American Invitational Mathematics Examination I, AIME I, 1990 based on fair coin.

## Fair Coin Problem – AIME I, 1990

A fair coin is to be tossed 10 times. Let i|j, in lowest terms, be the probability that heads never occur on consecutive tosses, find i+j.

• is 107
• is 73
• is 840
• cannot be determined from the given information

### Key Concepts

Integers

Combinatorics

Algebra

AIME I, 1990, Question 9

Elementary Algebra by Hall and Knight

## Try with Hints

5 tails flipped, any less,

by Pigeonhole principle there will be heads that appear on consecutive tosses

(H)T(H)T(H)T(H)T(H)T(H) 5 tails occur there are 6 slots for the heads to be placed but only 5H remaining, ${6 \choose 5}$ possible combination of 6 heads there are

$\sum_{i=6}^{11}{i \choose 11-i}$=${6 \choose 5} +{7 \choose 4}+{8 \choose 3}+{9 \choose 2} +{10 \choose 1} +{11 \choose 0}$=144

there are $2^{10}$ possible flips of 10 coins

or, probability=$\frac{144}{1024}=\frac{9}{64}$ or, 9+64=73.

Categories

## Positive divisor | AIME I, 1988 | Question 5

Try this beautiful problem from the American Invitational Mathematics Examination I, AIME I, 1988 based on Positive divisor.

## Positive Divisors- AIME I, 1988

Let $\frac{m}{n}$ in lowest term, be the probability that a randomly chosen positive divisor of $10^{99}$ is an integer multiple of $10^{88}$. Find m+n.

• is 107
• is 634
• is 840
• cannot be determined from the given information

### Key Concepts

Integers

DIvisors

Algebra

AIME I, 1988, Question 5

Elementary Number Theory by David Burton

## Try with Hints

$10^{99}=2^{99}5^{99}$

or, (99+1)(99+1)=10000 factors

those factors divisible by $10^{88}$

are bijection to number of factors $10{11}=2^{11}5^{11}$ has, which is (11+1)(11+1)=144

one required probability =$\frac{m}{n}=\frac{144}{10000}=\frac{9}{625}$

m+n=634.

Categories

## Ordered pair Problem | AIME I, 1987 | Question 1

Try this beautiful problem from the American Invitational Mathematics Examination I, AIME I, 1987 based on Ordered pair.

## Ordered pair Problem – AIME I, 1987

An ordered pair (m,n) of non-negative integers is called simple if the additive m+n in base 10 requires no carrying. Find the number of simple ordered pairs of non-negative integers that sum to 1492.

• is 107
• is 300
• is 840
• cannot be determined from the given information

### Key Concepts

Integers

Ordered pair

Algebra

AIME I, 1987, Question 1

Elementary Algebra by Hall and Knight

## Try with Hints

for no carrying required

the range of possible values of any digit m is from 0 to 1492 where the value of n is fixed

Number of ordered pair (1+1)(4+1)(9+1)(2+1)

=(2)(5)(10)(3)

=300.

Categories

## Arranging in column | AIME I, 1990 | Question 8

Try this beautiful problem from the American Invitational Mathematics Examination I, AIME I, 1990 based on Arranging in column.

## Arranging in column – AIME I, 1990

In a shooting match, eight clay targets are arranged in two hanging columns of three targets each and one column of two targets. A marks man is to break all the targets according to the following rules

1 ) The marksman first chooses a column from which a target is to be broken,

2 ) the marksman must then break the lowest remaining target in the chosen column. If the rules are followed, in how many different orders can the eight targets be broken?

• is 107
• is 560
• is 840
• cannot be determined from the given information

### Key Concepts

Integers

Arrangement

Algebra

AIME I, 1990, Question 8

Combinatorics by Brualdi

## Try with Hints

Let the columns be labelled A,B and C such that first three targets are A, A and A the next three being B, B and B and the next being C and C in which we consider the string AAABBBCC.

Since the arrangement of the strings is one-one correspondence and onto to the order of shooting for example first A is shot first, second A is shot second, third A is shot third, first B is shot fourth, second B is shot fifth, third B is shot sixth, first C is shot seventh, second C is shot eighth,

or, here arrangement of the strings is bijective to the order of the shots taken

the required answer is the number of ways to arrange the letters which is $\frac{8!}{3!3!2!}$=560.

Categories

## Head Tail Problem | AIME I, 1986 | Question 13

Try this beautiful problem from the American Invitational Mathematics Examination I, AIME I, 1986 based on Head Tail Problem.

## Head Tail Problem – AIME I, 1986

In a sequence of coin tosses, one can keep a record of instances in which a tail is immediately followed by a head , a head is immediately followed by ahead and etc. We denote these by TH, HH, and etc. For example in the sequence TTTHHTHTTTHHTTH of 15 coin tosses we observe that there are two HH, three HT, four TH and five TT subsequences. How many different sequences of 15 coin tosses will contain exactly two HH, three HT, four TH, and five TT subsequences?

• is 107
• is 560
• is 840
• cannot be determined from the given information

### Key Concepts

Integers

Combinatorics

Algebra

AIME I, 1986, Question 13

Elementary Algebra by Hall and Knight

## Try with Hints

Let us observe the sequences.

H switches to T three times, T switches to H four times.

There are 5 TT subsequences.

We are to add 5 T’s into, the string. There are already 4 T’s in the sequence.

We are to add 5 balls in 4 urns which is same as 3 dividers ${5+3 \choose 3}$=56

We do the same with 2H’s to get ${2+3 \choose 3}$=10

so, $56 \times 10$=560.

Categories

## Problem on Positive Integers | PRMO-2019 | Problem 26

Try this beautiful problem from Algebra PRMO 2019 based on Positive Integers.

## Positive Integers | PRMO | Problem 26

Positive integers x,y,z satisfy xy+z=160 compute smallest possible value of x+yz.

• 24
• 50
• 29
• 34

### Key Concepts

Algebra

Integer

sum

PRMO-2019, Problem 26

Higher Algebra by Hall and Knight

## Try with Hints

x+yz=$\frac{160-z}{y}$+yz

=$\frac{160}{y}+\frac{z(y^{2}-1)}{y}=\frac{160-z}{y}+\frac{zy^{2}}{y}=\frac{160-z}{y}+zy$

for particular value of z, $x+yz \geq 2\sqrt{z(160-z)}$

or, least value=$2\sqrt{z(160-z)}$ but an integer also

for least value z is also

case I z=1, $x+yz=\frac{159}{y}+y$ or, min value at y=3 which is 56

case II z=2, $x+yz=\frac{158}{y}+2y$ or, min value at y =2 which is 83 (not taken)

case III z=3, $x+yz=\frac{157}{y}+3y$ or, min value at y=1 which is 160 (not taken)

case IV z=4, $x+yz=\frac{156}{y}+4y$ or, min at y=6 which is 50 (taken)

case V z=5, $x+yz=\frac{155}{y}+5y$ or, minimum value at y=5 which is 56 (not taken)

case VI z=6, $x+yz=\frac{154}{y}+6y$ $\geq 2\sqrt{924}$>50

smallest possible value =50.

Categories

## Interior Angle Problem | AIME I, 1990 | Question 3

Try this beautiful problem from the American Invitational Mathematics Examination I, AIME I, 1990 based on Interior Angle.

## Interior Angle Problem – AIME I, 1990

Let $P_1$ be a regular r gon and $P_2$ be a regular s gon $(r \geq s \geq 3)$ such that each interior angle of $P_1$ is $\frac{59}{58}$ as large as each interior angle of $P_2$, find the largest possible value of s.

• is 107
• is 117
• is 840
• cannot be determined from the given information

### Key Concepts

Integers

Polygons

Algebra

AIME I, 1990, Question 3

Elementary Algebra by Hall and Knight

## Try with Hints

Interior angle of a regular sided polygon=$\frac{(n-2)180}{n}$

or, $\frac{\frac{(r-2)180}{r}}{\frac{(s-2)180}{s}}=\frac{59}{58}$

or, $\frac{58(r-2)}{r}=\frac{59(s-2)}{s}$

or, 58rs-58(2s)=59rs-59(2r)

or, 118r-116s=rs

or, r=$\frac{116s}{118-s}$

for 118-s>0, s<118

or, s=117

or, r=(116)(117)

or, s=117.

Categories

## Smallest positive Integer Problem | AIME I, 1990 | Question 5

Try this beautiful problem from the American Invitational Mathematics Examination I, AIME I, 1990 based on smallest positive integer.

## Smallest positive Integer Problem – AIME I, 1990

Let n be the smallest positive integer that is a multiple of 75 and has exactly 75 positive integral divisors, including 1 and itself. Find $\frac{n}{75}$.

• is 107
• is 432
• is 840
• cannot be determined from the given information

### Key Concepts

Integers

Divisibility

Algebra

AIME I, 1990, Question 5

Elementary Number Theory by David Burton

## Try with Hints

75=$3 \times 5^{2}$=(2+1)(4+1)(4=1)

or, $n=p_1^{a_1-1}p_2^{a_2-1}…..$ such that $a_1a_2….=75$

or, 75|n with two prime factors 3 and 5

Minimizing n third factor =2

and factor 5 raised to least power

or, $n=(2)^{4}(3)^{4}(5)^{2}$

and $\frac{n}{75}=(2)^{4}(3)^{4}(5)^{2}$=(16)(27)=432.

Categories

## Proper divisors | AIME I, 1986 | Question 8

Try this beautiful problem from the American Invitational Mathematics Examination I, AIME I, 1986 based on Proper divisors.

## Proper Divisor – AIME I, 1986

Let S be the sum of the base 10 logarithms of all the proper divisors (all divisors of a number excluding itself) of 1000000. What is the integer nearest to S?

• is 107
• is 141
• is 840
• cannot be determined from the given information

### Key Concepts

Integers

Divisors

Algebra

AIME I, 1986, Question 8

Elementary Number Theory by David Burton

## Try with Hints

1000000=$2^{6}5^{6}$ or, (6+1)(6+1)=49 divisors of which 48 are proper

$log1+log2+log4+….+log1000000$

$=log(2^{0}5^{0})(2^{1}5^{0})(2^{2}5^{0})….(2^{6}5^{6})$

power of 2 shows 7 times, power of 5 shows 7 times

total power of 2 and 5 shows=7(1+2+3+4+5+6)

=(7)(21)=147

for proper divisor taking out $2^{6}5^{6}$=147-6=141

or, $S=log2^{141}5^{141}=log10^{141}=141$.

Categories

## Combinatorics in Tournament | AIME I, 1985 | Question 14

Try this beautiful problem from the American Invitational Mathematics Examination I, AIME I, 1985 based on Combinatorics in Tournament.

## Combinatorics in Tournament- AIME I, 1985

In a tournament each player played exactly one game against each of the other players. In each game the winner was awarded 1 point, the loser got 0 points, and each of the two player earned $\frac{1}{2}$ point if the game was a tie. After the completion of the tournament, it was found that exactly half of the points earned by each player were earned against the ten players with the least number of points. (In particular, each of the ten lowest scoring players earned half of her/his points against the other nine of the ten). What was the total number of players in the tournament?

• is 107
• is 25
• is 840
• cannot be determined from the given information

### Key Concepts

Integers

Combinatorics

Algebra

AIME I, 1985, Question 14

Elementary Algebra by Hall and Knight

## Try with Hints

Let there be n+10 players

Case I from n players not in weakest 10, ${n \choose 2}$ games played and ${n \choose 2}$ points earned

Case II n players also earned ${n \choose 2}$ points against weakest 10

Case III now weakest 10 played among themselves ${10 \choose 2}$=45 games and 45 points earned

Case IV 10 players also earned 45 points against stronger n

So total points earned= 2[${n \choose 2}$+45]=$n^{2}-n+90$

case V 1 point earned per game ${n+10 \choose 2}$=$\frac{(n+10)(n+9)}{2}$ games and $\frac{(n+10)(n+9)}{2}$ points earned

So $n^{2}-n+90=\frac{(n+10)(n+9)}{2}$

or, $n^{2}-21n+90=0$

or, n=6, n=15 here taking n>10,

or, n=15 or, n+10=25.