each term is obtained by adding 4 to the preceding one

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imago: Junior | Next Rank: 30 Posts; Posts: 17; Joined: Sun Sep 09, 2007 3:34 am; Location: spain

each term is obtained by adding 4 to the preceding one

by imago » Tue Sep 18, 2007 4:07 pm

In a sequence, each term is obtained by adding 4 to the preceding one. If the sum of the first 10 terms is equal to 80, what is the result of the addition of the first 40 terms?

94
320
2,720
27,200
54,400

OA:C

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Source: Beat The GMAT — Problem Solving | Tue Sep 18, 2007 4:07 pm

kajcha: Master | Next Rank: 500 Posts; Posts: 321; Joined: Tue Aug 28, 2007 5:42 am; Thanked: 1 times

by kajcha » Tue Sep 18, 2007 5:21 pm

Ans C

This is AP. Sum of n terms in AP is given by

S(n) = n/2(2a+(n-1)d)

First part of q says

80 = 10/2(2a+(10-1)4) => a = -10

Now S(40) = 20(2*-10+(40-1)4) = 2720