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Re: What is the sum of the multiples of 7 from 84 to 140, inclus [#permalink]
13 Feb 2015, 11:20

If it doesn't occur to us to use the (first+last)/2 we can always go up 7, starting with 84 until 140. Adding them up wouldn't take longer. That if panic hits and we cannot think of the easiest way, which I guess would be "(first+last)/2".

(first+last)/2 --> gives the mean. we know that 9 is the N.

So, we then use: M= S/N --> 112 = S / 9 --> S = 9*112 --> S = 1008.

gmatclubot

Re: What is the sum of the multiples of 7 from 84 to 140, inclus
[#permalink]
13 Feb 2015, 11:20

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