Question

The sum of the first 1 million primes is N. Without knowing N's value, one can determine that the ones'digit of N cannot be

Answer 1

All primes are odd, except two. Therefore the sum of the first million primes is one even number plus 999999 odd numbers. Since odd*odd = odd, the first million primes must be even + odd = odd.

An odd number obviously has an odd final digit, so the only option that can be firmly rules out is b (which is even).