Which is the better buy?
A 24 oz box of cereal for $3.69 or a 36 oz box of cereal for $4.89?

We must solve two division problems...

$3.69 ÷ 24 ≈ $0.15

$4.89 ÷ 36 ≈ $0.14

Which is the better buy?
A 18 oz jar of jam that costs $1.99 or a 24 oz jar of jam that costs $2.29?

$1.99 ÷ 18 ≈ $0.11

$2.29 ÷ 24 ≈ $0.10

Often it is necessary to use 0.5¢ when comparing unit prices, especially when the unit prices are similar ($0.12 vs. $0.125).  Sometimes a half cent makes the difference in a bargain!