Look at the screenshots-- ? ? the sooner the better! explain if you

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Sam buys milk at a lower rate, at $3.40 per gallon.

If you look at the graph, you can see that Y is the cost and X is the amount of milk. Look at x = 2, and you'll see that the line makes y = 7. That means the rule is: p = 3.5g, where p is y and g is x. You can easily see that the first option with Edwin is more expensive than Sam.