Explanation:
the old story :
1+2+3+...+n =
n+1
+
(n-1) + 2 = n+1
+
(n-2) + 3 = n+1
...
you see we sum up the largest and the smallest number. then we go one step in on both sides and sum these 2 numbers. another step in on both sides.
and so on.
how often can we do that ? well, n/2 times.
the beauty is, that this works also with odd numbers, as an odd number plus 1 is even, and then that can be divided by 2.
so, the sum is
n×(n+1)/2
now, our n = 49.
so,
the sum is
49×50/2 = 49×25 = 1225
I don't think you wrote the answer options correctly.
so, please pick the one equal to 1225.