Max Maths, Year 5, Learn together, Multiplying by 19 or 21 mentally (1)

In an engaging lesson designed for Year 5 students, the focus is on mental multiplication strategies, specifically multiplying by 19 or 21. The lesson builds on the understanding that students already know how to multiply by 20, which is a foundational skill for this new concept. The technique taught involves a two-step process; for instance, when multiplying 19 by 8, students start by multiplying 20 by 8 to get 160. However, since the aim is to multiply by 19, not 20, the next step is to subtract one lot of 8 from the total. This adjustment brings the result down to 152, demonstrating that 19 times 8 equals 152. This method allows students to perform what might seem like complex multiplications in a simple and efficient manner.

The lesson encourages students to think critically about the numbers they are working with and to use subtraction as a tool to refine their multiplication results. By understanding that multiplying by 19 is the same as multiplying by 20 and then subtracting one set of the multiplicand, students can confidently tackle these calculations without the need for pen and paper. This mental arithmetic skill is not only practical but also helps to enhance students' numerical fluency and their ability to manipulate numbers with ease. The lesson's approach is both intuitive and logical, making the process of multiplying by 19 or 21 accessible and less intimidating for young learners.