amathews wrote:

A farmer can plow the corn field in 12 days. After working for 5 days, his son joins him and together they finish plowing the field in 4 days. How many days would it take the son to plow the corn field alone?

A) 9 days

B) 10 days

C) 11 days

D) 13 days

E) 16 days

proper methodFarmer does work in 12 days so his ONE day work= 1/12..

work left after 5 days = 1-5/12=7/12..

If 7/12 of work is finished in 4 days, entire work could be finished by both = 4*12/7=48/7..

Thus one day work of both combined=7/48..

So one day work of son = \(\frac{7}{48}-\frac{1}{12}=\frac{7-4}{48}=\frac{1}{16}\)..

Therefore son can complete the work in 16 days..

Faster and logical methodFarmer works for 5+4 or 9 days...

If he completes entire work in 12 days, he will finish 9/12 or 3/4 in 9 days..

Remaining work, 1-3/4=1/4, is done by son in 4 days..

So if son does 1/4 in 4 days, he will complete the work in 4*4=16 days.

E

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1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372

2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html

3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html

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