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Bunuel
What is the greatest value of q such that 9^q is a factor of 21! ?

A. 1
B. 3
C. 4
D. 5
E. 6
\(9 = 3^2\)

HIghest power of 3 in 21! is 9

21/3 = 7
7/3 = 2

So, the highest power of 9 will be 1/2*9 => 4

Hence, correct answer will be (C) 4
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Bunuel
What is the greatest value of q such that 9^q is a factor of 21! ?

A. 1
B. 3
C. 4
D. 5
E. 6

9=3^2
largest power of 3 in 21! is 21/3 + 21/9 = 7+2=9
9^q= 3^9
q=9/2 = 4
C
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9^q=3^2q
To find the answer we it is needed to find the number of powers of 3 in 21!, that equals to 21/3^1 = 7; 21/3^2=2
7+2=9 number of powers of 2 in 21!
Since initial condition is that 3^2q, 9=2q; q=9/2=4,5, therefore maximum power of 3 in 21! is 4. Answer C is correct.
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Could you explain why we are dividing 9 by 2 ?
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What is the greatest value of q such that 9^q is a factor of 21! ?

A. 1
B. 3
C. 4
D. 5
E. 6

Since 9 = 3^2, we need the power of 3 in the prime factorization of 21!.

21/3 gives 7 factors of 3.
21/9 gives 2 additional factors of 3.

So the power of 3 in the prime factorization of 21! is 7 + 2 = 9.

Since 9^q = 3^(2q),

2q <= 9

Thus, the greatest possible integer value of q is 4.

Answer: C.

Mayzone hope it's clear.
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Mayzone
Could you explain why we are dividing 9 by 2 ?

Also check out this post: https://anaprep.com/number-properties-highest-power-in-factorials/
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