If you subtract the sum of the first 10 positive integers from the sum of the first 20 positive integers, the first 10 positive integers will cancel out:
(1 + 2 + 3 + 4 + 5 + …+20) – (1 + 2 +3 + 4 + 5 +…+10)
= 1 + 2 + 3 + 4 + 5 + …+20 – 1 – 2 – 3 – 4 – 5 -… -10
= (1 – 1) + (2 – 2) + … + (10 –10) + (11 + 12 + 13 + 14 +… + 20)
=11 + 12 + 13 + 14 + 15 + 16 + 17 + 18 + 19 + 20
= 155
(A)