Suppose that the number in the head of the person who displays 1 1 is X, and the number in the head of the person who displays 8 is Y. Because their average value is 10, x+y=20. Suppose the person who displays 9 thinks Z, then x+z= 14, Y+Z = 8; You can solve it by connecting equations.

X= 13, y=7 and z= 1 are obtained.

Of course, the unknowns can also be set in order;

If only two unknowns are learned, z is expressed as (14-x), which is the same. Solve the equations x+y=20 and (14-x)+y=8.

The key is to use the average value to calculate.