First find the total cost of the electricity: $4 / watt * 300 watts = $1200. Then add the late fee: $1200 + $150 = $1350.
This justifies what answer for what question? Q & A:
Darla needs to pay $4/watt of electricity for 300 watts of electricity, plus a $150 late fee. How much does she pay in total?
1350