What is the sum of odd integers from 35 to 85, inclusive?

[BTGModeratorVI]

What is the sum of odd integers from 35 to 85, inclusive?

A) 1,560
B) 1,500
C) 1,240
D) 1,120
E) 1,100

Answer: A
Source: Official guide

[Brent@GMATPrepNow]

What is the sum of odd integers from 35 to 85, inclusive?

A) 1,560
B) 1,500
C) 1,240
D) 1,120
E) 1,100

Answer: A
Source: Official guide

We want: 35 + 37 + 39 + 41 + . . . . 79 + 81 + 83 + 85

Add the numbers in pairs, starting from the outside and working towards the middle, we get:
35 + 37 + 39 + 41 + . . . . 79 + 81 + 83 + 85 = (35 + 85) + (37 + 83) + (39 + 81) + . . .
= (120) + (120) + (120) + (120) + .....
= 120 times some integer

So, the desired sum must be DIVISIBLE by 120
Only one answer choice is divisible by 120

Answer: A

Cheers,
Brent

[Scott@TargetTestPrep]

What is the sum of odd integers from 35 to 85, inclusive?

A) 1,560
B) 1,500
C) 1,240
D) 1,120
E) 1,100

Answer: A
Source: Official guide

The sum of the odd integers from 35 to 85, inclusive, is:

sum = avg x quantity

sum = (85 + 35)/2 * (85 - 35)/2 + 1

sum = 120/2 * 50/2 + 1

sum = 60 * 26 = 1560

Answer: A