Ifaequals the sum of the even integers from 2 to 40, inclusive, andbequals the sum of the odd integers from 1 to 39, inclusive, what is the value of a - b ?	Select the correct option.
A) 1 B) 10 C) 19 D) 20 E) 21

Rationale: this is a solution from beatthegmat : even numbers : ( 40 - 2 ) / 2 + 1 = 20 even integers . ( 40 + 2 ) / 2 = 21 is the average of the even set . sum = avg * ( # of elements ) = 21 * 20 = 420 = a odd numbers : ( 39 - 1 ) / 2 + 1 = 20 odd integers . ( 39 + 1 ) / 2 = 20 is the average of the odd set . sum = avg * ( # of elements ) = 20 * 20 = 400 = b a - b = 420 - 400 = 20 . answer : d
Answer: D