The sum of the numbers from 1 to n is n(n+1)2.
In this problem, n=102016.
Instead of multiplying everything out, we could realize that 102016 is even, so 102016+1 is odd (and therefore will not have any 2's in its prime factorization).
Now we only need to find the number of 2's in 102016, and get rid of one of them because we divide by 2 in the fraction (102016)(102016+1)2.
102016=52016∗22016, so we now know that there are 2016 twos in 102016. We divide the expression by 2, so one less than 2016 is which 2015.
So, there are 2015 twos in the prime factorization of the sum, and it is our answer. I hope this helps.