Answer:
-140
Explanation:
You can simplify the entire problem by dividing by 3 so 3(1-2+3-4+5...)
By doing this you'll realise every other number is counting up by one when you calculate it and every even number reciprocates the number but as a negative:
1-2 = -1
-1+3 =2
2 - 4 = -2
And so you will also realise you can simplify every 2 numbers by -1 since 1-2 = -1, 3-4 = -1.... so you will do that 50 times to reach -50 and since it will simplify to 3(-50) the answer will be -150.
Hope you understand.