A, (Lao Zhang's age is equal to Xiao Li's present age) Xiao Li's age

Xiao Li's age now.

C. Lao Zhang's current age

D, (when Xiao Li's age is equal to Lao Zhang's present age) Lao Zhang's age

The difference between two adjacent ages is equal to their present age difference.

If C is regarded as the unit of 1, then A is 1/5, indicating that the age difference between them is equal to (1- 1/5) ÷ 2 = 2/5.

Because the age difference between them is constant, D is equal to 1+2/5 = 7/5.

Therefore, Lao Zhang's present age is equal to 72 ÷ (1+7/5) = 30 years old, and Xiao Li's age is equal to 30 × (1-2/5) = 18.

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