A alone can do a certain job in 25 days while B alone can do the same in 20 days. A started the work and was joined by B after 5 days. The number of days taken to complete the entire work will be:
Explanation:
A taken 25 days alone and B takes 20 days alone to complete a certain task.
Let the work to be done = LCM (25, 20) = 100 units
∴ Efficiency of A = 100/25 = 4 units/day, and Efficiency of B = 100/20 = 5 units/day
First 5 days A was working alone, hence work done by A = 5 × 4 = 20 units.
Remaining work = 100 - 20 = 80 units
If A & B work together, they can complete (5 + 4 =) 9 units/day; so to complete 80 units they will take 80/9 days
Total time required = 80/9 + 5 = 125/9 days
Hence, option (c).
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