A and B together can dig a trench in 12 days, which A alone can dig in 28 days, B alone can dig it in :
Q. A and B together can dig a trench in 12 days, which A alone can dig in 28 days, B alone can dig it in :
Answer: 21 days
Notes: B's 1 day's work = (A + B)'s 1 day's work - A's 1 day's work $latex \therefore \frac{1}{12}-\frac{1}{28} = \frac{7-3}{84}&s=1$ $latex = \frac{4}{84} = \frac{1}{21}&s=1$ ∴ Required time = 21 days. Hence option [B] is correct answer.