Fives everywhere

I have a favorite number, which so happens to be a positive integer, and it satisfies the following properties:

  • It can be expressed as the product of 5 prime numbers .

  • It has 5 proper divisors.

  • The sum of digits is also 5.

What is my favorite number?


The answer is 32.

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1 solution

Kushal Bose
Dec 4, 2016

Let the positive integer is n n .It can be expressed as product of 5 5 primes.So n = p 1 p 2 p 3 p 4 p 5 n=p_1 p_2 p_3 p_4 p_5 .

No of proper divisor is 5 5 .So ,total divisors 5 + 1 = 6 5+1=6 .Each power of primes is 1 1 Then total number if facotrs will be 2 5 = 32 2^5=32 .

6 = 2 × 3 = 1 × 6 6=2 \times 3=1 \times 6 .pwers of 2 1 2-1 and 3 1 3-1 will not be allowed because then it cannot be expressed of 5 5 primes.So power ( 6 1 ) (6-1) will be accepted which also be expressed as product of 5 primes.Now n = p 5 n=p^5 .The smallest n will be n = 2 5 = 32 n=2^5=32 which also satisfies third condition.

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