Answer: a) 1
Explanation:
Let's look at the experienced painter. He can get 1 wall completed in 20 minutes --> which means he can get 3 walls done in 1 hour.
The inexperienced painter can get 1 wall done in 1 hour.
How much did they get done in 1 hour?
Experienced + Inexperienced
3 walls + 1 wall
= 4 walls
So, together they can paint 4 walls in 1 hour.