Answer:
 y=2.25x
Explanation:
 4.50 ÷ 2 = 2.25 per pound
 11.25 ÷ 5 = 2.25 per pound
22.50 ÷ 10 = 2.25 per pound
Proportional relationship as each pound cost $2.25, and will increase accordingly per pound.
constant of proportionality: 2.25
pounds: x
 cost: y
y = kx
y = 2.25x