Burgers and Shakes (Solve multi-step word problems using rounding.)

Sally and her friends want to buy 8 cheeseburger meals with $80, where each meal costs $9. By rounding the cost to $10, they determine they have enough money. The exact cost is $72, leaving them with $8. They use this to buy 4 milkshakes at $2 each. The lesson teaches solving multi-step word problems using rounding and basic arithmetic operations.

How many milkshakes can they buy with the remaining money?

What mathematical operations did you use to solve the problem?