Answer: Sal used a debit card and paid a total of $324.80 for the stereo. (I took the test too)
Step-by-step explanation: Multiply $21.30 by 18 months. Then subtract the interest, $58.60, from the product because Sal paid immediately with a card which implies using a debit card. You get $324.80.
48, 53, 58, 63 is from low to hight numbers
4, 8, 12, 16, 20, 24, 28, 32, 36, 40
Hope this helps! ^_^