Word problem involving billing

I am not sure how to set this problem up. Any help would be appreciated.

Sally received a bill for a total of 8 hours labor on the repair of her bulldozer. She paid $50 to the master mechanic and $90 to his apprentice. If the master mechanic gets $10 more per hour than his apprentice, then how many hours did each work on the bulldozer.

THIS IS WHAT I CAME UP WITH THUS FAR.

We write T for the number of hours the master mechanic

worked, T' for the number of hours the apprentice worked, P for the price

the master charges per hour, and P' for the price the apprentice charges

per hour.

Four variables. We need four equations.

First Equation: Express the fact that the total number of hours worked is

8. (What does this have to do with T, T', P and P'? It involves only two of

these variables.)

Second Equation: Express the fact that the master mechanic earned $50.

Third Equation: Express the fact that the apprentice earned $90.

Fourth Equation: Express the fact that the master's pay rate exceeds the

apprentice's pay rate by $10 per hour.