a can do a certain job in 25 days which b alone..., Time & Work, General Mathematics

Question:A can do a certain job in 25 days which B alone can do in 20 days. A started the work and was joined by B after 10 days. The number of days taken in completing the work was:

A `12 1/2` 
B `14 2/9` 
C 15 
D `16 2/3`

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+ AnswerD

+ Explanationsolution:
Work done by A in 10 days = `(1/25 xx 10)`
                          = `2/5`.
Remaining work = `(1 - 2/5)`
               = `3/5`.
`(A +B)`'s 1 day's work = `(1/25 + 1/20)`
                        = `9/100`.
Now, `9/100` work is done by them in 1 day.
`:. 3/5` work will be done by them in `(100/9 xx 3/5) = 20/3` days.
Total time taken = `(10 + 20/3)`
                 = `16 2/3` days.

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