Puzzle ZBWT 

Workings

Hint
How much of a wall does Sam build in one day?

Answer
4 days.

Reasoning
In one day Sam can build 1/20 of a wall, Taylor and Kelly can each build 1/10.

So in one day, in total, they can build 1/20 + 1/10 + 1/10 = 1/4.

Therefore the wall will take 4 days to build.