Here the unit rate is the rate per one pound. Because Marta paid $3.48 for 3 pounds it makes sense to divide $3.48 by 3 to get the unit rate. We can verify this by using a proportional equation. Based on the fact that the per pound price is the same for one pound as it is for there pounds, and we don't know what this rate is let's create a variable p for the unit rate (price per pound).
Our equation is
$3.48/3 = p/1 multiplying both sides by 3 tells us $3.48 = 3p to solve for p we divide $3.48 by 3
and we find the unit price (per pound price) = $1.16
The constant of proportionality would be the weight (in pounds) of turnips bought (in this case c = 3)
if we make c = constant of proportionality, p = unit rate, and d = total cost in dollars
than d = cp or cost in dollars is proportional to the unit rate ($1.16 per pound) times the constant of proportionality (weight purchased).
