Answer :
We can solve this simply by multiplying 20 by each number (4, 5, 3, 6)
$20 X 4= $80---- $80 - $1.50 - $1.50= $77. Does that make $117? No, so we move on
$20 X 5= $100 (now if you want a short cut, there's your answer right there. You can stop doing the work because automatically, $100 is lower than $117 and we know that number can only go lower, not higher)
So the answer is 6 hours
$20 X 6= $120----$120 - $1.50 - $1.50 = $117.00