Explanation:

Let’s break it down:

- A + B + C can finish the work in 20 days. So in 1 day, together, they do 1/20 of the work.

- A + B can do it in 30 days, so in 1 day, they get through 1/30 of it.

- If you subtract (A+B)’s 1-day work from (A+B+C)’s 1-day work, you get C's solo 1-day work: 1/20 - 1/30 = (3-2)/60 = 1/60.

- So, C alone will finish in 60 days.

Now, for the options:

1. 10 days – Way too quick, not realistic given the group speeds.

2. 30 days – Still too fast for just C alone, since A+B need 30.

3. 45 days – Closer, but C’s real pace is slower.

4. 60 days – That's the one that fits the math and logic.

So, Option 4 (60 days) is the accurate answer.