a and b can together finish a work in 30 days..., Time & Work, General Mathematics

Question:A and B can together finish a work in 30 days. They worked at it for 20 days and then B left. the remaining work was done by A alone in 20 more days. A alone can finish the work in :

A 48 days 
B 50 days 
C 54 days 
D 60 days

/47

+ AnswerD

+ Explanationsolution:
Work done by A and B in 20 days = `(1/30 xx 20)`
                                                      =`2/3`.
Remaining work =`(1 - 2/3)`
                           =`1/3`.
Now, `1/3` work is done by A in 2o days
`:.` Whole work will be done by A in `(20 xx 3) = 60` days.

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