A is twice as good a workman as B and together they finish a piece of work in 13 days. In how many days will B alone finish the work?

39

Given,

A is twice as good a workman as B, therefore,

⇒ A : B = 2 : 1   (Efficiency)  

⇒ They both completed the work in 13 days = (2 + 1) x 13 = 39 units,  ..(Efficiency × Days = Total work)

⇒ Time required for B to complete the work alone = \(\frac{39}{1}\) = 39 days.    ..(\(\frac{Work}{Efficiency}\) = Time)