GMAT Prep?? (Sum of even)

VP_Jim: GMAT Instructor; Posts: 1223; Joined: Thu May 01, 2008 3:29 pm; Location: Los Angeles, CA; Thanked: 185 times; Followed by:15 members

by VP_Jim » Thu May 15, 2008 9:19 pm

Hi dferm,

There's a really quick way to solve these. Note that the average of all the even numbers between 100 and 200 is going to be about 150. Then, note that we have about 50 even numbers between 100 and 200.

So, if we take 150 * 50, we get 7500, which is close to the right answer (close enough!).

Jim S. | GMAT Instructor | Veritas Prep

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sandeep_chhabra: Senior | Next Rank: 100 Posts; Posts: 73; Joined: Fri Feb 01, 2008 1:28 am; Location: India; Thanked: 1 times; Followed by:1 members

by sandeep_chhabra » Thu May 15, 2008 9:51 pm

AP can be easily used:

n/2 [2a + (n-1)d]

n=Total even numbers between 102 & 200 = 50
a=first term =102
d=2

=>25 [204+(49)2]
=>25[302]
=>7550

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bia: Senior | Next Rank: 100 Posts; Posts: 30; Joined: Wed May 07, 2008 6:16 am

by bia » Thu May 15, 2008 9:56 pm

We got the formula to get the sum of even integers:

S = (a1 + an)x(number of integers)/2

Apply to this question :
a1=102; an=200
number of integers = ((200-102)/2) + 1 = 50

S= (102+200)x50/2 = 7550

Bia

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