Answer:
$0.57 per bun
1 3/4 buns per dollar
Explanation:
You have a price given as $2 for 3 1/2 buns and want to know the associated unit rate.
Unit rate
The unit rate of A per B is found by dividing the A quantity by the B quantity. This tells you the amount of A you get for 1 unit of B.
Application
When two numbers are given and a "unit rate" asked for, it is not always clear which of the numbers is the A quantity, and which is the B quantity. The unit rate can be figured either way:
dollars per bun = ($2)/(3 1/2 buns) = 4/7 dollars per bun ≈ $0.57 /bun
buns per dollar = (3 1/2 buns)/($2) = 1 3/4 buns per dollar
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Additional comment
Where time is involved, we often want the rate specified as per unit time.
Where money is involved, we often want the rate specified as currency per unit of item (dollars per bun). This helps us compare costs.
Rates with the units reversed can be just as useful for comparing costs or times: 1 3/4 buns per dollar is a better value than 1 1/3 buns per dollar, for example.
Sometimes, rates must be inverted to be useful in a given problem. A typical case is the "working together" problem, where rates are often given in terms of completion time per project. The "working together" rates can only be added in terms of projects per time, the inverse of the given rates.