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Bunuel
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power of 2 in 7! -- \(\frac{7}{2}\) - \(\frac{3}{2}\) - 1 -- 3+1 = 4 -- in \((7!)^2\) = 4*2 = 8

power of 3 in 7! -- \(\frac{7}{3}\) - 2 -- 2 -- in \((7!)^2\) = 2*2 = 4

power of 5 in 7! -- \(\frac{7}{5}\) - 1 -- 1 -- in \((7!)^2\) = 1*2 = 2

x = 8, y = 4, and z = 2

x+y+z = 8+4+2 = 14

D is the answer
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7!^2 = (1*2*3*4*5*6*7)^2 = (2^4 * 3^2 *5^1*7^1)^2

so x= 8
y= 4
z = 2
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Bunuel
If x, y and z are largest positive integers for which \(2^x3^y5^z\) is a factor of \((7!)^2\), what is the value of \(x + y + z\) ?

A. 7
B. 12
C. 13
D. 14
E. 16

Since 7! = 7 x 6 x 5 x 4 x 3 x 2 = 7 x 2 x 3 x 5 x 2^2 x 3 x 2 = 2^4 x 3^2 x 5 x 7, then we see that (7!)^2 = 2^8 x 3^4 x 5^2 x 7^2. Therefore, x = 8, y = 4, z = 2 and hence x + y + z = 14.

Answer: D
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Solving it by writing out 7! makes sense to me, but I'm wondering why I got the wrong answer when I used the formula for prime factors of a factorial.


(7/2)+(7/4)+(7/8) (throw out remainder) = 3+1+0 = 4
(7/3)+(7/9) (throw out remainder) = 2+0 = 2
(7/5) + (7/25) (throw out remainder) = 1 + 0 = 1

4+2+1 = 7

So I got 7, but the answer should be 14? I'm confused.


Edit: I realized my error. The question says (7!)^2... I reread it so many times and didn't even notice the ^2... ugh...
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