X alone can do a piece of work in 15 days. Y alone can do the same work in 30 days. X, Y and Z, working together, can complete the work in nine days. In how many days Z alone can complete the same work?
Answer & explanation
Correct answer: option 2
X = 15 days, Y = 30 days, X + Y + Z = 9 days,
⇒ X + Y + Z = 10 (Efficiency)
⇒ 6 + 3 + Z = 10
⇒ Z = 10 - 9
⇒ Z = 1,
Therefore, time required for Z alone to complete the work = \(\frac{90}{1}\) = 90 days, ..(\(\frac{Work}{Efficiency}\) = Time)