Answer: 8.5
Step-by-step explanation: We know constant of proportionality is just a fancy name for slope. The slope formula is y2-y1 divided by x2-x1. Here the price of the buckets is y and the number of buckets is x. So using 42.50 and 25.50 as y2 and y1 you would do 42.50-25.50=17, and using 3 and 5 as x2 and x1, you would do 5-3=2. Now since we have both parts of the equation you would do 17 which is y2-y1 divided2 which x2-x1 and you would get 8.5