A number is said to be a nice number if it has exactly 4 factors (Including one and number itself). let then number of factors. How many nice numbers are there?
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We know that if a number is of the form
n = p 1 a 1 × p 2 a 2 × p 3 a 3 . . . . × p n a n ... where a i s are primes,
then the number of it's factors is ( a 1 + 1 ) ( a 2 + 1 ) ( a 3 + 1 ) . . . ( a n + 1 ) ... this is because in the factor, the power of prime p j could be anything from 0 to a j implying j + 1 different powers and talking about all primes together will give that number of factors.
Thus if you want a number to have 4 factors exactly, it HAS to be of the form p 3 where p is prime, or of the form p 1 × p 2 where p 1 and p 2 are primes.
Here you have 5 choices of primes, and they are 2 , 3 , 5 , 7 , 1 1 so you have to chose 2 out of them and have power 1. So there are ( 2 5 ) = 1 0 numbers.
And other way, the cubes can only be 2 3 and 5 3 , so there will be 2 more numbers.
That accounts to give total 1 2 numbers.