Final answer:
The constant proportionality between the number of tres leches cakes and the number of cups of milk needed is 3.5.
Step-by-step explanation:
The equation given is y = 3.5x, where y represents the number of cups of milk needed and x represents the number of tres leches cakes. The constant proportionality between x and y can be found by dividing y by x. In this case, the constant of proportionality is 3.5.
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