Explanation:

- P can complete the work in 18 days. This means P does 1/18 of the work each day.

- Q can complete the work in 36 days. Thus, Q does 1/36 of the work daily.

- Together, their combined work per day is:

- (1/18) + (1/36) = (2/36) + (1/36) = 3/36 = 1/12 of the work.

- Hence, both P and Q together take 12 days to complete the work.

Option 2: 12

- Option 1: 24 days - Too long, as they work together.

- Option 3: 20 days - Also too long for joint efforts.

- Option 4: 10 days - Slightly short; overestimates their speed.