#### Answer

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

#### Work Step by Step

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.