C: (400-20x)(10+x)
x would represent how many dollar increases there was in the price and if 20 less are sold per dollar increase it would have to be (400-20x)(10+x)
to explain more. let’s say the price increases by 5 dollars. that would make x=5. so it would be (400-100) because 100 less shirts are sold. and (10+5) because you’re adding $5 to the price. If this doesn’t make sense lmk and I can re explain it
