P and Q working alone can complete a work in 9 days and 18 days respectively. They work on alternate days one at a time. P works on the first day. In how many days the whole work will be completed?
Show answer & explanation
Correct answer: C
Explanation
Given:
P works alone = 9 days
Q works alone = 18 days
P works on the first day
P's rate per day = 1/9
Q's rate per day = 1/18
Work done in 2 days = 1/9 + 1/18 = 1/6
Work will be completed in 6 such cycles (of 2 days each)
Total time = 6 × 2 = 12 days