chumway motors discounts the car cost by 10% and then it takes off 20% of the new price. What discount does this represent from the original price of the car?

a) 28%

b) 30%

c) 40%

d) 72%

i got d) because i set a fake price($1000)...and then did the 10%(new price: $900) discount and then the 20%(np: $720) discount. then i did 720/1000, and got 72%.

the answer is a)....i think i did the percentage difference wrong...right????