An 18-pound bag of dog food is $27. What is the constant of proportionality that relates cost in dollars, y, to the number of pounds of dog food, x ?

Constants of proportionality are involved with direct variation. Direct variation takes the form y = kx where y is the dependent variable; k is the constant of proportionality; and x is the independent variable. The formula for this problem is: 27 = k(18). When you divide both sides by 18, you find that k = 3/2.
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