The equation for direct variation is y = kx
k is the constant of variation.
15 = k(3)
15/3 = k
k = 5
The constant is 5
Answer: y = 5x
7•x=x-15
I think this would work, if I understood what you were saying correctly.
2.50 is the answer because if she has 2 then she can get 1
72×3.14=226.08 millimeters