Explanation

The rate at which Masons build walls is what we need to understand with this problem.

15*X = 20

15*X/20 = 1 Wall

Therefore, a fifteenth of a wall can be built by one mason in 20 days.

Each day, a mason can build 1/(15*20) or 1/300th of an entire wall.

Therefore, it would take one mason 300 days to build a wall on their own.

It would take 15 masons 20 days (300/20) to build a wall.

And we can estimate the 12 day solution with the following formula:

300/12 = 25

It would take 25 masons 12 days .