A pen and a pencil set cost $56 if the pen were to go on sale for half the price the cost of the pen and pencil set would be $35 what is the price of the pencil alone Please hurry in 10 minutes

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.