If you would like to know how long will take you to paint one wall, you can calculate this using the following steps:
1/3 of a wall ... 1/4 hour
1 wall ... x hours = ?
1/3 * x = 1/4 * 1
1/3 * x = 1/4 /*3
x = 1/4 * 3
x = 3/4 hour
Result: It will take you 3/4 hour to paint one wall.