Chapter P, Prerequisites - Section P.9 - Modeling with Equations - P.9 Exercises - Page 70: 32

The assistant worked 35 hours and the plumber worked 70 hours.

Let's say that the assistant worked $t$ hours. Then the plumber worked $2*t$ hours. The final bill was \$4025: $4025=25*t+45*2t$ We solve for $t$: $4025=115*t$ $t=4025/115=35$ Thus the assistant worked 35 hours and the plumber worked 35*2=70 hours.