Difference between revisions of "2009 AIME I Problems/Problem 3"

Latest revision as of 02:35, 16 January 2022

Problem

A coin that comes up heads with probability $p > 0$ and tails with probability $1 - p > 0$ independently on each flip is flipped $8$ times. Suppose that the probability of three heads and five tails is equal to $\frac {1}{25}$ of the probability of five heads and three tails. Let $p = \frac {m}{n}$ , where $m$ and $n$ are relatively prime positive integers. Find $m + n$ .

Solution 1

The probability of three heads and five tails is $\binom {8}{3}p^3(1-p)^5$ and the probability of five heads and three tails is $\binom {8}{3}p^5(1-p)^3$ .

$\begin{align*} 25\binom {8}{3}p^3(1-p)^5&=\binom {8}{3}p^5(1-p)^3 \\ 25(1-p)^2&=p^2 \\ 25p^2-50p+25&=p^2 \\ 24p^2-50p+25&=0 \\ p&=\frac {5}{6}\end{align*}$

Therefore, the answer is $5+6=\boxed{011}$ .

Solution 2

We start as shown above. However, when we get to $25(1-p)^2=p^2$ , we square root both sides to get $5(1-p)=p$ . We can do this because we know that both $p$ and $1-p$ are between $0$ and $1$ , so they are both positive. Now, we have:

$\begin{align*} 5(1-p)&=p \\ 5-5p&=p \\ 5&=6p \\ p&=\frac {5}{6}\end{align*}$

Now, we get $5+6=\boxed{011}$ .

~Jerry_Guo

Solution 3

Rewrite it as : $(P)^3$ $(1-P)^5=\frac {1}{25}$ $(P)^5$ $(1-P)^3$

This can be simplified as $24P^2 -50P + 25 = 0$

This can be factored into $(4P-5)(6P-5)$

This yields two solutions: $\frac54$ (ignored because it would result in $1-p<0$ ) or $\frac56$

Therefore, the answer is $5+6$ = $\boxed {011}$

Video Solution

https://youtu.be/NL79UexadzE

~IceMatrix

Video Solution 2

https://www.youtube.com/watch?v=P00iOJdQiL4

~Shreyas S

@@ Line 18: / Line 18: @@
 == Solution 2 ==
-We start as shown above. However, as we get to <math>25(1-p)^2=p^2</math>, we square root both sides to get <math>5(1-p)=p</math>. We can do this because we know that both <math>p</math> and <math>1-p</math> are between <math>0</math> and <math>1</math>, so they are both positive. Now, we have:
+We start as shown above. However, when we get to <math>25(1-p)^2=p^2</math>, we square root both sides to get <math>5(1-p)=p</math>. We can do this because we know that both <math>p</math> and <math>1-p</math> are between <math>0</math> and <math>1</math>, so they are both positive. Now, we have:
 <cmath>\begin{align*}
@@ Line 31: / Line 31: @@
 == Solution 3 ==
-Rewrite it as : <math>(P)^3</math><math>(1-P)^5</math>=<math>\frac {1}{25}</math> <math>(P)^5</math><math>(1-P)^3</math>
+Rewrite it as : <math>(P)^3</math><math>(1-P)^5=\frac {1}{25}</math> <math>(P)^5</math><math>(1-P)^3</math>
 This can be simplified as <math>24P^2 -50P + 25 = 0</math>
-This can be factored into: <math>(4P-5)(6P-5)</math>
+This can be factored into <math>(4P-5)(6P-5)</math>
-This yields two solutions: <math>\frac54</math>(ignored because it would result in <math>1-p<0</math>) and <math>\frac56</math>
+This yields two solutions: <math>\frac54</math> (ignored because it would result in <math>1-p<0</math>  ) or <math>\frac56</math>
-Therfore, the answer is <math>5+6</math> = <math>\boxed {011}</math>
+Therefore, the answer is <math>5+6</math> = <math>\boxed {011}</math>
 ==Video Solution==

2009 AIME I (Problems • Answer Key • Resources)
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