Question

Tracy can wallpaper 7 rooms in a new house in 21 hours. together with her trainee they can wallpaper the 7 rooms in 12 hours. how long would it take the trainee working by herself to do the​ job? round to the nearest tenth.

Answer 1

The first thing we must do for this case is to define a variable.
 We have then:
 x: time it takes the trainee to do the work alone.
 For this case we have the following equation:
 7/21 + 7 / x = 7/12
 Solving for x we have:
 x = 28 hours
 Answer:
 it would take the trainee working by herself about:
 x = 28 hours