Explanation:
16.
x = Xien's age
2 + x/2 = 11
x/2 = 9
x = 18
so, Xien is 18 years old.
17.
4x - 5 = 7
first dividing everything by 4 is not a mistake. I would have done it as second step simply to avoid dealing with the fractions. but it works either way.
so, his way :
4x/4 - 5/4 = 7/4
4x/4 = 12/4
x = 3
my way
4x - 5 = 7
4x = 12
x = 3