Explanation

A is twice as good as B, so A's work rate is 2 times that of B.

Since A takes 30 days less than B, let's assume B takes x days to complete the work. Then, A takes x - 30 days to complete the same work.

Since A is twice as good, A's work rate is 2 times that of B, so A completes the work in half the time it takes B. Therefore, x - 30 = (1/2)x

Solving the equation, we get x = 60 days for B and x - 30 = 30 days for A.

Since A and B are working together, their combined work rate is the sum of their individual work rates. So, the total work is completed in (60 * 30) / (60 + 30) = 1800 / 90 = 20 days.

Therefore, the correct answer is: 20 days