ratio of common differences

[2010gmat]

1. Find the ratio of commn differences d1 and d2 of two APs if the respective nth terms are in the ratio of 2n+3:n-11.

a. 1:2
b. 2:3
c. 2:1
d. 1:3
e. can't be determined

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2. find the sum upto 20 terms

1/12 + 1/20 + 1/30 + 1/42 + ----

a. 5/96
b. 20/69
c. 20/96
d. 20/23
e. 10/69

[2010gmat]

No takers for these ques??

they seemed easy to me but i could not solve these 2 ques even after 20-25 mins...

experts pls help

[adam15]

what "d1 and d2 of two APs" stands for?
but I tried to solve the first problem as follow:
I equated each answer to the ratio 2n+3:n-11, and solve the one that yield an integer is the right answer. e.g
2n+3:n-11=1:3, after computation it yields 4.
Would you please post the answer for both.
which is the best answer
1:3

[2010gmat]

d1 and d2 stand for respective common differences..

can u pls elaborate your approach?? i will post the OA soon...

i have solved the second question ... OA is 20/69