Because the right 1 question is given 5 points, the wrong question is deducted 3 points, and the difference between right and wrong questions is 5+3 = 8 points. Assuming that 20 questions are all right 100, he gets 84 points, with a difference of 100-84 = 16, and a difference of 8 points between right and wrong questions. Then, 16÷8 = 2, and he made two mistakes.