X and Y together can finish a piece of work in 15 days, while Y alone can finish it in 40 days. X alone can finish the work in:

24 days 26 days 25 days 23 days

24 days

X + Y = 15 days, Y = 40 days, ⇒ X + Y = 8,  (Efficiency) ⇒ X + 3 = 8, ⇒ X = 5, ⇒ Time required by X to complete the work alone = \(\frac{120}{5}\) = 24 days,   ..(\(\frac{Work}{Efficiency}\) = Time)