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:

42

A + B = 21 days, A = 42 days,

⇒ Efficiency of A + B = 2, 

⇒ 1 + B = 2

⇒ B = 1,

⇒ Time required by B to complete the work alone = \(\frac{42}{1}\) = 42 days,    ..(\(\frac{Work}{Efficiency}\) = Time)