**Topic**

Ha, Chau, Lien and Ngan went to buy milk tea together. All four of you have a total of 185 thousand VND. Four friends buy four cups of milk tea for 35,000 VND a cup. You order more pearls for milk tea. A portion of black pearls costs 5 thousand dong, a portion of white pearls costs 10 thousand dong. Let x, y be the number of black and white pearls, respectively, that the four of you plan to buy more.

a) Write a first-order inequality for two unknowns x, y to show how much you can afford to pay for the black and white pearls.

b) Show an integer solution of the inequality.

**Detailed explanation**

a) The amount to buy four cups of milk tea is: 35.4 = 140 (thousand dong).

The amount when adding x part of black pearl is: 5x (thousand dong).

The amount when adding y portion of white pearls is: 10y (thousand dong).

The total amount that the four of you have to pay for 4 cups of milk tea and extra pearls is:

5x + 10y + 140 (thousand dong).

Since the four of you have a total of 185 thousand dong, 5x + 10y + 140 185

⇔ x + 2y 9

So the first-order inequality of two unknowns x, y to represent the amount you can afford to pay for the black and white pearls is x + 2y ≤ 9.

b) The pair (x;y)=(1; 1) is an integer solution of the given inequality because:

1 + 2.1=3 ≤ 9.