Number of perfect square factors greater than 1

CAT 2019 Slot 2 · QA · Medium · Number theory

How many factors of 24×35×1042^4 \times 3^5 \times 10^4 are perfect squares which are greater than 1?

[TITA]

Answer

44

Explanation

First, write NN in prime factorized form: N=24×35×(2×5)4=28×35×54N = 2^4 \times 3^5 \times (2 \times 5)^4 = 2^8 \times 3^5 \times 5^4

Any factor is of the form 2a×3b×5c2^a \times 3^b \times 5^c. For a factor to be a perfect square, a,b,ca, b, c must all be even integers.

  • Possible values for a{0,2,4,6,8}a \in \{0, 2, 4, 6, 8\} (5 choices)
  • Possible values for b{0,2,4}b \in \{0, 2, 4\} (3 choices)
  • Possible values for c{0,2,4}c \in \{0, 2, 4\} (3 choices)

Total perfect square factors =5×3×3=45= 5 \times 3 \times 3 = 45. Excluding 1 (where a=b=c=0a=b=c=0), the number of perfect square factors greater than 1 is 451=4445 - 1 = 44.

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