GMAT PREP QUESTION

Question

Please help with the following question:

lagomez · Answer

1. 7x + 77y = 350
7(x+y) = 350
x+11y = 50

x + y = total number of terms, or n

subtract both equations and you get
10y = 50 - n

Therefore n must be 40 to get an integer

2. 350-77 = 273
273/7 = 39
39+1 = 40

blakemancillas · Answer

Thanks! I got x + 11y = 50 and derived that (x + y) = n but didn't think to solve them as a system of equations. Helpful

mikioso · Answer

dont use the algebric way on this one. just think about it: if there were only sevens the answer could be 50 (350/7). now you now you have to use one of the 77's. 77 equals 11x7 so 50-11=39 terms of seven and the 40th term is 77 or the opposite. anyway a(n)=40

skovinsky · Answer

Hi!

This question is a great illustration of the power of logical thinking on the GMAT. The more flexible you are in your approaches, and the less you bind yourself to algebraic solutions, the quicker and more confidently you'll move through the quant section.

We see that the units digit of each term is 7. We see that the units digit in our sum is 0. How can a bunch of 7s add up to a 0?  Only if the total number of terms is a multiple of 10 .

Only one answer choice is a multiple of 10: choose (c).

blakemancillas · Answer

Great insight! Thanks!

smartmundu · Answer

thanks skovinsky

How i did: 350 means there must be 50 7's, now if i want to have one 77 term then i must remove 11 7's, so this become 39 7's + 1 77, total of 40

Divyadisha · Answer

Suppose there are only 7, then n = 350/7= 50

To have 1 '77', we need 11 '7's'

so the set will have

39 7's and 1 77's (n=40)
28 7's and 2 77's (n= 30)
17 7's and 3 77's (n= 20)
6 7's and 4 77's (n=10)

C is the answer

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