reSolve Bakery: Lesson 2

Australian curriculum number (ACMNA100)

Lesson abstract

This lesson generalises the repeated doubling strategies of Lesson 1 to flexible use of the properties for multiplication. Students find the number of cupcakes in 40 trays by multiplying by its factors (first 4 and then 10). This is a practical use of the associative property of multiplication. In a second inquiry, students again find, present and compare strategies and undertake consolidation exercises based around making multiplication easy by flexibly multiplying by factors in turn.

Mathematical purpose (for students)

Splitting numbers into their factors can help with multiplying.

Mathematical purpose (for teachers)

The central idea in this lesson is that multiplying by a number can be done by multiplying by its factors, so for example, multiplying by 40 is the same as multiplying by 4 and then by 10. This principle is illustrated by arrays (now in grid form) and revealed in two inquiries, one calculating 24 x 40 in various ways and the next calculating 32 x 25.  There are applications to making mental multiplication easy (e.g. the easy way to multiply by 25).