Let me just write this simply.
Joe's parts: $25 Â Â Â Â Â Â Joe's hourly rate for brakes: $50/hr
Kathy's parts:$70 Â Â Â Â Â Kathy's hourly rate: $40/hr
x = time (to fix the brakes)
So both will start with an initial charge for parts. Joe: $25, and Kathy: $70
Then for each hour, your price goes up...
Here is a table.
        0 hrs    1 hr    2 hrs    3 hrs    4 hrs    5hrs    6 hrs
Joe       25     75     125     175     225    275     325
Kathy     70     110     150     190     230    270    310
So look at the table, notice how joe's prices always end in 5, and Kathy's always end in 0. Because neither Joe's or Kathy's rates go up by a number that ends in 5, they will never be the same.
Answer: never