Answer 1

This program calculates the sum of primes 1 through n entered by the user. The trickiest part for me was getting the test for prime right.

For any number A, we loop though all the numbers B from 2 up to A/2.
if A % B != 0 then A is prime. That is to say if any A cannot be divided evenly by any number from 2 up to A/2 then A is prime:

https://code.sololearn.com/#cpp