Answer: 14 liters
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Step-by-step explanation:
Draw a vertical line up from x = 7 until you reach the diagonal red line. Mark point P at this location. Then from point P, move horizontally until you reach the y axis. You should get to y = 14.
See the diagram below.
The equation of this red line is y = 2x. Whatever the input x is, we double it to get y. So if x = 7, then y = 2x = 2*7 = 14
This means that Joe drinks 14 liters of milk in 7 days. Or you could say his rate is 14 liters per week.