Suppose he did the X problem correctly, but he didn't do 0.5*(20-x) correctly.

5x-2*0.5*(20-x)=64

The solution is x= 14.

Solution 2: Binary linear equation

Did the right X, did the wrong Y, and did the wrong Y.. According to the question, there are

x+2y=20 ( 1)

5x-2y=64 (2)

From (1)+(2)

6x=84

X= 14 y=3。