Explanation:

- Let's first understand the problem in terms of work rate:

- When P, Q, and R work together, they finish the task in 15 days.

- P and R together finish the task in 40 days.

- From this, we can say:

- The work rate of P + Q + R = 1/15 of the task per day.

- The work rate of P + R = 1/40 of the task per day.

- To find Q's work rate:

- Q's work rate alone = (P + Q + R) - (P + R)

- = 1/15 - 1/40

- = 8/120 - 3/120

- = 5/120

- = 1/24 of the task per day

- Therefore, Q alone can finish the task in 24 days.

- The correct option is: Option 2: 24 days