If x is directly proportional to y, and x = 2 when y = 5, find y when x = 7.

Correct answer: 35/2. For direct proportion, x₁/y₁ = x₂/y₂, so 2/5 = 7/y₂ and y₂ = 35/2.

Background: In direct proportion, when one quantity increases, the other increases in the same ratio.

• 2/35: 2/35 is the inverted value.
• 5/2: 5/2 does not satisfy the proportion.
• 35/4: 35/4 does not satisfy the proportion.

Did you know? The ratio x/y stays constant at 2/5.

Memory trick: x/y = constant.