Max Maths, Year 5, Learn together, Multiplying by 19 or 21 mentally (2)

In an engaging lesson for Year 5 students, the focus is on enhancing mental arithmetic skills by learning how to multiply by 19 or 21. Building on the foundation of multiplying by 20, students can easily progress to multiplications involving 21. The method is straightforward and involves a two-step process. For example, when tasked with calculating 21 multiplied by 13, students first calculate 20 times 13, which equals 260. This is a familiar step as they have already mastered multiplying by 20.

However, since the aim is to multiply by 21, students recognise that they have one group of 13 less than required. To rectify this, they simply add another 13 to the initial result. Hence, 260 (which is 20 times 13) plus 13 equals 273. Therefore, 21 multiplied by 13 equals 273. This method reinforces the concept of building on known facts (multiplying by 20) and makes it easier for students to perform mental calculations with numbers just above 20, such as 21, enhancing their numerical fluency and mental math strategies.