Answer:
20 tiles
Explanation:
Given that,
number of walls needed to be painted = 3
volume of one shower wall = 4 x 6 x 7
 = 168 feet³ 
volume of 3 shower wall = 3 x 168 
 = 504 feet³
1 foot = 12 inches 
504 foot³ = 6048 inches³
volume 4x4 tiles will cover for 3 walls = 3(4 x 4 x 7)
 = 336 inches³
The number of tiles to fill the area = 6048 inches³/ 336 inches³
 = 18 tiles
10% more than the number of tiles to fill the area in case any tiles break or need to be cut, or replaced later
18 + (18 x 10/100) 
= 19.8
≈ 20 tiles