Q) X and Y can do a piece of work in 20 days and 12 days respectively. X started the work alone and then after 4 days Y joined him till the completion of the work. How long did the work last?

a) 6 days                     b) 10 days             c) 15 days                     d) 20 days

Ans:

X’s 1 day work = 1/20

Y’s 1 day’s work = 1/12

∴ (X + Y)’s 1 day’s work = 1/20 + 1/12 = 8/60 = 2/15

X’s 4 days work = 4 x 1/20 = 1/5

∴ Balance work = 1 – 1/5 = 4/5

Since time taken by (X + Y) to do 2/15 work = 1 day

∴ Time taken by A to do 4/5 work = (4/5)/(2/15) = 6 days

Therefore, total time taken = 4 + 6 = 10 days

Hence, Answer is [B].

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