Answer
we need to use the formula for direct variation, which is:
y = kx
where y is the cost in dollars
x is the number of gallons
k is the constant of proportionality.
We can use the given information to set up an equation for the relationship between y and x:
13.56 = k(6)
Solving for k, we can divide both sides by 6:
k = 13.56/6
k = 2.26
Therefore, the constant of proportionality that relates the cost in dollars, y, to the number of gallons, x, is 2.26.