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.

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
 
 it would take the trainee working by herself about:
 x = 28 hours