The sum of the first two natural numbers, each having 15 factors (including 1 and the number itself), is 

It is given that the number of factors of these two numbers is 15. Also, the factors of 15 are 1, 3, 5, and 15.

The number of factors of N is given by the formula ⇒  , where    (i.e. N is a product of powers of prime numbers.)

Hence, the value of N will be least when  are as close as possible and the primes themselves are the least distinct prime numbers.

∴ let  and  (i.e. two smallest prime numbers)

Also, the  should be 3 and 5 respectively because we need them to be as close as possible

⇒   ⇒ 

⇒   ⇒ 

Now, calculate the lowest value of N and the second lowest value of N.

Hence, the sum of first two natural numbers (each having 15 factors) =