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.