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Option 3 : 371⁄2 days
Formula used:
Time is taken = Total work/Efficiency
Calculation:
A completes (4/5)th of the work in 20 days
The whole completed by A in (20 × 5)/4 = 25 days
Remaining work = 1 - (4/5) = 1/5
(1/5) of the work done by A and B in 3 days
Whole work completed by A and B in (3 × 5) = 15 days
Now, 1-day work of B alone = (1/15) - (1/25)
⇒ 2/75
The whole work is done by B alone = 75/2 days
∴ The total number of days taken by B alone to do work is 371⁄2 days
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