a b and c can do a piece of work in 11..., Time & Work, General Mathematics

Question:A,B and C can do a piece of work in 11 days, 20 days and 55 days respectively, working alone. How soon can the work be done if A is assisted by B and C on alternate days ?

A 7 days 
B 8 days 
C 9 days 
D 10 days

/47

+ AnswerB

+ Explanationsolution:
(A + B)'s 1 day's work `= (1/11 + 1/20)`
                               `= 31/220`.
(A + C)'s 1 day's work `= (1/11 + 1/55)`
                              `= 6/55`.
Work done in 2 days `= (31/220 + 6/55)`
                            `= 55/220`
                            `= 1/4`.
Now, `1/4` work is done in 2 days.
`:.` Whole work will be done in `(4 xx 2)`
                                           `= 8` days.

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