Answer:
First mechanic worked 20 hours and second mechanic worked 5 hours
Step-by-step explanation:
Let the number of hours the first mechanic worked = 
Let the number of hours the second mechanic worked = 
Therefore, we can write 2 equations and then solve them simultaneously:

 
Rearranging the first equation: 
and substituting into the second equation to find  :
:


Now sub  into the first equation to find
 into the first equation to find 

Therefore, first mechanic (a) worked 20 hours and second mechanic (b) worked 5 hours