The 42.21 would represent the money spent on paintbrushes. This because the amount of money spent on paintbrushes doesn't change (it's a set amount). You can tell this by the fact that 42.21 is not being multiplied by anything (the 42.21 cannot change at all)
The 21.64x is the cost of paint per gallon.
Cost of paint per gallon means that the cost changes depending on many gallons you are getting.
The 21.64 represents the cost of 1 gallon of paint, and the 'x' represents the number of gallons.
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So the Answer is option 2:
$42.21 on paintbrushes
Paint costs $21.64 per gallon