If A and B together can do a piece of work in 5 days, and A alone can do the same work in 10 days, then in how many days can B alone do the piece of work?

10 days

A + B = 5 days,  A = 10 days,

⇒ A + B = 2   (Efficiency)

⇒ 1 + B = 2,

⇒ B = 1,

⇒ Time required by B to complete the work alone = \(\frac{10}{1}\) = 10 days, ..(\(\frac{Work}{Efficiency}\) = Time)