To find: The plumber and his assistant working hours.
The assistant works for 35 hours and plumber works for 70 hours.
Given:
The labor for plumber is $45 and for assistant is $25.
Total bill for labor charge is $4025.
Calculation:
Let the assistant working hours be x.
Tabulate the given information into the language of algebra.
In words  In Algebra 
Assistant working hours  x 
Plumber working hours  2x 
Assistant charges  25x 
Plumber working charges 

Model the equation for the above information.
Simplify the above equation for x,
Plumber working hours,
Thus, the assistant works for 35 hours and plumber works for 70 hours.