A can complete a piece of work in 12 days, and B can complete the same work in 36 days. In how many days will both, together, complete the work?

7 8 6 9

9

A = 12 days, B = 36 days, ⇒ Time required by A + B to complete the total work = \(\frac{36}{3+1}\) = \(\frac{36}{4}\) = 9 days,   ..(\(\frac{Work}{Efficiency}\) = Time)