Question 896838
The unfinished section is 50*12 square feet;


Paint rate regarding usage is 1 gallon for 15 square feet.


{{{50*12*ft^2*(1/15)(gallons/(ft^3))}}}

{{{((50*12)/15)*gallons}}}


but you want the cost to finish; so one more conversion.
{{{((50*12)/15)*gallons*5.65(dollars/gallon)}}}


You can compute that.  I just kept most of the numbers with the resulting units so you see how this can work.