Detailed Solution: MADAHAEA/ WA=MBDBHBEB/ WB ----(1) Where, M = no of men D = no of days H = hours per day E = Efficiency DA=15 EA=0.8EB All other variables are equal. Using equation (1) 15×0.8×EB=DB×EB ⇒DB=15×0.8 ⇒DB=12 B alone can do the work in 12 days. Alternate Solution: Efficiency of A = 1/15 Let number of days taken by B to complete the work be D. Efficiency of B = 1/D According to the question, ⇒ 1/15 = 80/100 × 1/D ⇒ D = 80 × 15/100 = 12 days.