A and B can do a piece of work in 9 days and 18 days, respectively. As they were ill, they could do 90% and 72% of their efficiency, respectively. How many days will they take to complete the work together?

71/7
8
5
121/2

Correct Answer: 71/7

Explanation:

Because of illness,
A's 1 day's work = 90% of 1/9 = 1/10
B's 1 day's work = 72% of 1/18 = 1/25
∴ (A + B)'s 1 day's work
= (1/10) + (1/25) = (5 + 2)/50 = 7/50
Hence, time taken by them to complete the work = 50/7 days
= 7¹/₇

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