AlgebraEasy–MediumSolving a two-variable equation for one value
Algebra practice question
Sandwiches cost $4 and drinks cost $2 at a stall. The equation 4x + 2y = 40 relates the number of sandwiches x and drinks y that can be bought for exactly $40. If 6 sandwiches are bought, how many drinks are bought?
Answer: 8
Substitute x = 6: 24 + 2y = 40, so 2y = 16 and y = 8.
Common wrong answers
- If you answered 4
- 4 comes from 24 + 2y = 40 solved as 2y = 8 (subtracting in the wrong direction).
- If you answered 16
- 16 is 2y, the money spent on drinks, not the number of drinks.
- If you answered 20
- 20 is 40/2, ignoring the sandwiches.
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