The oil for your car comes in three sizes – a little 50-ounce bottle on sale for $7.99, a medium 100-ounce bottle for $13.99, and a big 150-ounce bottle on sale for $17.99. So, which one should you purchase to get the best deal?

By using proportional reasoning, we can compare the price per ounce and determine the most cost-effective. The phrase “price per ounce” tells us exactly which ratios we should compare – the price divided by ounces.

Little bottle: $7.99/50 ounces is about $0.16 per ounce. Medium bottle: $13.99/100 ounces is about $0.14 per ounce. Big bottle: $17.99/150 ounces is about $0.12 per ounce. Therefore, the big bottle is the best deal.