Answer:
2 gallons of paint
Explanation:
If 3/4 of a gallon covers 2/5 of a wall, this means that a half of 3/4 gallons can cover 1/5 of a wall.
By knowing this, you can just make up your own expression
(the trick is to find a way to break it down to one, so you can multiply and manipulate it to save yourself some brain cells)
Hoped this helped you :3