Explanation:

Let’s break it down:

- A does the whole job in 20 days, so does 1/20 of the job per day.

- B does it in 30 days, or 1/30 per day.

- Together, in one day: 1/20 + 1/30 = (3+2)/60 = 5/60 = 1/12 of the work daily.

- In 6 days together: 6 × 1/12 = 1/2 of the total work gets done.

- A leaves, so B works alone for 2 days: 2 × 1/30 = 1/15 of the work.

- Total done so far: 1/2 + 1/15 = (15+2)/30 = 17/30.

- Remaining work is 1 - 17/30 = 13/30.

- Suppose total time to complete is the same as if only A & B work together: their combined time is 12 days, since together they do 1/12 per day.

- Already, 6 (together) + 2 (just B) = 8 days have passed. Time left: 4 days.

Now, C joins B for those last 4 days:

- B alone does 1/30 per day.

- Let’s say C does x units per day. So in four days, combined, they do 4 × (1/30 + x) = 13/30.

- Set up the equation: 4 × (1/30 + x) = 13/30 ? (4/30) + 4x = 13/30

- 4x = (13/30 - 4/30) = 9/30 ? 4x = 9/30 ? x = 9/120 = 3/40

- C does 3/40 per day, A does 1/20 per day.

- 3/40 / 1/20 = (3/40) × (20/1) = 60/40 = 1.5 ? so C is 50% as efficient as A, or more efficient, to be precise.

So Option 3, 50%, is correct.

Option 3: 50% is the right answer.