Problem: Write a program to find sum of all multiples of 3 and 5 upto input n. If a number is multiple of both 3 and 5 add it only once. We were given this problem at work. I tried to “solve” it using math. Clearly, we can add all the multiples of 3 and 5 upto n and subtract multiples of 15 once to account for the overcounting. We could derive formula for this as follows: