When doing this problem, think about it first: the average height of three people =?

Average height of three people = total height of three people ÷3

What about the total height of three people?

Xiaohong =a

Xiao Qiang =a-7

Xiaoming =a+4

Then the average height of three people =[A+(A-7)+(A+4)]> 3 = A- 1.