Suppose Meng Qiang made a mistake in question X.
Equation according to the meaning of the question:
5*(20-x)-2x=86
100-5x-2x=86
7x= 14
x=2
He made two mistakes.
Method 2: (Arithmetic)
Analysis: If you do the wrong question, you will get 5+2=7 (points) less.
(5*20-86)÷(5+2)= 14÷7=2 (road)
He made two mistakes.