Difference between revisions of "2020 AIME II Problems/Problem 1"

Revision as of 22:32, 10 June 2020

Problem

Find the number of ordered pairs of positive integers $(m,n)$ such that ${m^2n = 20 ^{20}}$ .

Solution

First, we find the prime factorization of $20^{20}$ , which is $2^{40}\times5^{20}$ . The equation ${m^2n = 20 ^{20}}$ tells us that we want to select a perfect square factor of $20^{20}$ , $m^2$ . $n$ will be assigned by default. There are $21\times11=231$ ways to select a perfect square factor of $20^{20}$ , thus our answer is $\boxed{231}$ .

~superagh

Solution 2 (Official MAA)

Because $20^{20}=2^{40}5^{20}$ , if $m^2n = 20^{20}$ , there must be nonnegative integers $a$ , $b$ , $c$ , and $d$ such that $m = 2^a5^b$ and $n = 2^c5^d$ . Then $2a + c = 40$ and $2b+d = 20$ The first equation has $21$ solutions corresponding to $a = 0,1,2,\dots,20$ , and the second equation has $11$ solutions corresponding to $b = 0,1,2,\dots,10$ . Therefore there are a total of $21\cdot11 = 231$ ordered pairs $(m,n)$ such that $m^2n = 20^{20}$ .

Video Solution 2

https://youtu.be/Va3MPyAULdU

~avn

Video Solution

https://www.youtube.com/watch?v=x0QznvXcwHY

~IceMatrix

@@ Line 15: / Line 15: @@
 <cmath>2b+d = 20</cmath>
 The first equation has <math>21</math> solutions corresponding to <math>a = 0,1,2,\dots,20</math>, and the second equation has <math>11</math> solutions corresponding to <math>b = 0,1,2,\dots,10</math>. Therefore there are a total of <math>21\cdot11 = 231</math> ordered pairs <math>(m,n)</math> such that <math>m^2n = 20^{20}</math>.
+==Video Solution 2==
+https://youtu.be/Va3MPyAULdU
+~avn
 ==Video Solution==

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