Explanation:

- The work completed by A, B, and C together in 30 days is one unit.

- Their efficiency ratio is 5:3:8, meaning collectively their work rate is 5 + 3 + 8 = 16 units per day.

- In one day, A, B, and C together do 1/30th of the work. Thus, their combined efficiency equates to 1/30 units/day.

- 1 unit of work = 1/30 units x 30 days = 30/30 = 1 unit

- A and B's combined efficiency is 5 + 3 = 8 units per day.

- A and B work together for 20 days: 8 units/day x 20 days = 160 units.

- Total work is 480 units in 30 days. They complete 160/480 = 1/3 of the work.

- Remaining work for C is 1 - 1/3 = 2/3 of the work.

- C's efficiency = 8 units/day.

- Time for C to complete 2/3 of work = 2/3 divided by 1/60 = 40 days.

- Option 1: 40 days is correct.