Let Lao Li's age be X and Xiao Zhang's age be Y:

From the meaning of the question, we know that:

x-y=30

(x+y)÷2=42

x=30+y

x+y=42x2=84

(30+y)+y=84

30+2y=84

2y=84-30

y = 54 \2

y=27

x=30+y

=30+27

=57

Lao Li is 57 years old and Xiao Zhang is 27 years old.

For reference.