The pizza is cut into equal slices. You work out how many slices a fraction is: a quarter of 12 slices is 12 : 4 = 3 slices.
“Of the pizza” means the fraction of the whole pizza, “of what is left” only of what is still there. Go eater by eater and subtract every time.
If the question is how many slices the pizza had, work backwards: which fraction is left? The slices left are worth exactly that fraction.
Example
For parents
What does this exercise practice?
Fractions are experienced here as parts of a whole, once of the whole and once of what is left. Working backwards prepares equations and proportions.
Common pitfalls
The most common mistake: with “of what is left” the fraction is still taken from the whole pizza. Another one: a third of the pizza is mixed up with a third of the slices that remain.
How to practice at home
Share a real pizza, a cake or a bar of chocolate by fractions and count what is left.
Related: Fractions, Fraction of a quantity, Comparing fractions