We need to calculate the ratio $/h:
$21.75/3h = 7.25 $/h
$50.75/7h = 7.25 $/h
$65.25/9h = 7.25 $/h
$94.25/13h = 7.25 $/h
Now for 20h
7.25 $/h * 20h = 145 $
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