Series

[sanjana]

If each term in the sum a1+a2+a3...+an is either 7 or 77 and the sum is 350,which of the following could be n?
a)38
b)39
c)40
d)41
e)42

[xcusemeplz2009]

IMO C

7*39+77=350

n=39+1=40

OA pls

[Stuart@KaplanGMAT]

If each term in the sum a1+a2+a3...+an is either 7 or 77 and the sum is 350,which of the following could be n?
a)38
b)39
c)40
d)41
e)42

We can answer this one lickety split by looking at the units digit in the choices.

We know that the units digit in each term in the sum is 7. We also know that the sum ends in 0. The only way to get a bunch of 7s to add up to a 0 is if the number of terms is a multiple of 10.

Only (c) is a multiple of 10 - done!

[sanjana]

genius all the way!
Stuart,
I know a lot of ppl have asked u this.. Can i borrow your brain for 7th Nov 2009!!

Thanks