Answer:
4 hours
Step-by-step explanation:
Ratio of working together
Job : number of hours
1 : 3 = 1/3
Trainer
Job : number of hours
1 : x = 1/x
Trainee
Job : number of hours
1 : 3x = 1/3x
1/x + 1/3x = 1/3
Multiply through by 3x
3x(1/x) + 3x(1/3x) = 3x(1/3)
3 + 1 = x
4 = x
x = 4 hours
It takes the trainee 4 hours to finish the job