Discussion :: Time and Work

A does 80% of a work in 20 days. He then calls in B and they together finish the remaining work in 3 days. How long B alone would take to do the whole work?
Answer : Option C
Explanation :
Whole work is done by A in[\([20*\frac { 5 } {4 } ]\)= 25 days.
Now,[1\(\frac { 4 } { 5 } \)]i.e.,\(\frac {1 } { 5 } \)work is done by A and B in 3 days
Whole work will be done by A and B in (3 x 5) = 15 days.
A's 1 day's work =\(\frac {1 } { 25 } \),, (A + B)'s 1 day's work =\(\frac {1 } { 15 } \)
B's 1 day's work =[\(\frac {1 } { 15 } \)\(\frac {1 } { 25 } \)]=\(\frac {4 } { 150 } \)=\(\frac {2 } { 75 } \)
So, B alone would do the work in\(\frac {75 } { 2 } \)=37\( \frac { 1 } { 2 } \)days
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