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

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by BTGModeratorVI » Sat Apr 18, 2020 9:18 am

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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
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Source: — Problem Solving |

BTGModeratorVI wrote:
Sat Apr 18, 2020 9:18 am
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
Brent Hanneson - Creator of GMATPrepNow.com
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BTGModeratorVI wrote:
Sat Apr 18, 2020 9:18 am
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

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