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If each term in the sum a1+a2+a3+.....+an is either 7 or 77 and the sum equals 350

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If each term in the sum a1+a2+a3+.....+an is either 7 or 77 and the sum equals 350, which of the following could be equal to n? 

A. 38 
B. 39 
C. 40 
D. 41 
E. 42

What should be a better approach for such questions, is it number crunching or algebra. Please suggest

Official Answer

1 Answer

jaipster
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7x + 77y = 350, where x is # of 7's and y is # of 77's, so # of terms n equals to x+y;

7(x + 11y) = 350 -> x + 11y = 50 

let us try some number x = 39 and y = 1 which implies n = x + y = 40

hence the answer C

answered Jun 15, 2014 by Senior Associate (475 points)
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