Question 67

A and B can together complete a task in 21 days. If A alone takes 42 days to complete the task, then the number of days that B would take to do the task alone is:

Solution

A and B can together complete a task in 21 days. If A alone takes 42 days to complete the task.

LCM of 21 and 42 is 42 which is total work.

The efficiency of A = $$\frac{42}{42}$$ = 1 unit/day

The efficiency of A and B together = $$\frac{42}{21}$$ = 2 units/day

The efficiency of B = 2-1 = 1 unit/day

The number of days taken by B to do the task alone = $$\frac{42}{1}$$

= 42 days


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