Katie bought a sweater and 1 skirt that cost $20. The items she bought cost a total of $60. What was the cost of the sweater? ​

Sanderson is having trouble with his assignment. his shown work is as follows: − 3 7 7 3 = − 3 7 × 7 3 = − 21 21 = −1 however, his answer does not match the answer that his teacher gives him. complete the description of sanderson's mistake. find the correct answer.