A and B can build a certain wall in 20 days and 30 days respectively. In how many days can they together build two such walls?

To solve this problem, we first need to find out how much of the wall each person can build in one day.

1. A can build the wall in 20 days, so A can build 1/20 of the wall in one day.
2. B can build the wall in 30 days, so B can build 1/30 of the wall in one day.

When A and B work together, they can build (1/20 + 1/30) of the wall in one day.

To simplify this, we find a common denominator for 20 and 30, which is 60. So, we can rewrite the fractions as 3/60 for A and 2/60 for B.

So, A and B together can build (3/60 + 2/60) = 5/60 of the wall in one day.

This simplifies to 1/12 of the wall in one day.

Therefore, A and B together can build one wall in 12 days.

Since we want to know how long it will take them to build two walls, we simply multiply this by 2.

So, A and B together can build two walls in 12 * 2 = 24 days.