In mathematics, an equation is a statement that asserts the equality of two expressions, which are connected by the equals sign "=". The word equation and its cognates in other languages may have subtly different meanings; for example, in French an équation is defined as containing one or more variables, while in English, any equality is an equation.
Grace has 3 wheels of cheese and she wants to eat an equal amount each day for 12 days. So each wheel needs to last 12/3 = 4 days. So then she can eat 1 wheel/4 days = 1/4 wheel per day.