How to do a math problem?

(1) Buy X cups of coke and Y cups of milk tea.

2x+3y=20

X y is an integer, so y must be an even number.

When y=2, x=7.

When y=4, x=4.

When y=6, x= 1.

* * * There are three ways to buy.

(2)

Buy at least two cups of milk tea, y>2.

At least no one drinks, x+y > =8

Only the second of the above three combinations meets the conditions.

So everyone buys 4 cups of cola and 4 cups of milk tea.