Answer:
50
Explanation:
So you can think of this by grouping it like this:
(1-2) + (3-4) + (5-6) + ... + (97-98) + 99
which is equal to: (-1) + (-1) + (-1)... + (-1) + 99
(each group is equal to -1, and 99 won't have a pair since it's the last one)
then, find how many groups of -1 there are:
the groups start at 1 and end at 98, but there are two in each group, so 98/2 = 49. this means there are 49 groups.
so now, you know that there are 49 -1s, so 49 * (-1) = -49.
finally, you can't forget the extra 99 that didn't have a pair, so -49 + 99 = 50.