Answer:
88 hours on dump truck and 24 hours on fire engine.
The minimum cost will be $3480.
Explanation:
Constraints will be :

Plant A:

Plant B:

The Cost function is given as:

Now graphing this we will get the points (0,200) , (88,24) , (160,0)
So, the answer would be 88 hours on dump truck and 24 hours on fire engine.
And cost will be :

=

= $3480