Equation method: If Xiaoyu doesn't do X, he will do X wrong and answer (20-2X).

0×X+﹙﹣2﹚X﹢﹙20﹣2X﹚×5=64

X=3

20-2×3= 14

A: Xiaoyu answered 14 correctly, made 3 mistakes and didn't do 3.

Arithmetic method: Assume that it is all 20× 5 = 100 (minutes).

If the wrong question is not scored, 5+2 = 7 (points) will be deducted, and 7 points will be scored inside and outside (or the difference).

If you don't do the problem, if you get less than 5 points, you won't be deducted.

Did not do the problem+did the wrong problem 5+7= 12 (points). Internal and external difference 12.

Total score 100, actual score 64. The difference is 100-64=36 (points).

{This 36 points is a deduction due to not doing the questions and wrong questions, so}

36÷ 12=3 (Tao) There are three questions about doing wrong and not doing it.

The reverse is 20-3-3= 14 (Dao)

A: Xiao Yu answered 14.