The most he can buy for each cupcake is 1.25
Use point slope form to answer this
Y = x - 10
18=r/2
Multiply by 2 on each side,
36=r
r=36
Hope this helps.
Divide it into one piece so 3.6 / 2 = 1.8 then times that by the number of pieces you want so 3.6 / 2 * 5 = 9