Doubling & Halving with Multiplication

Doubling and halving works because of the relationship between multiplication and division. Since multiplication and division are opposites, we can multiply one factor by 2 and divide the other factor by 2 and the product does not change.

This method of multiplication works well with some numbers. Numbers that are multiples of 5 and 10 work well. Since it is more difficult to compute with fractions and decimals, it works best to take half of an even number. With some experience, students soon find which numbers work nicely with this method.

64 x 25 = ? 64 x 25 I can easily take half of 64 and double x 50 I can easily take half of 32 and double x 100 I can easily find this product! 16 x 100 = 1600 “COOL”

Try one on your own… Choose a problem from these: 33 x x 36 5 x 148

Did you choose this? 5 x 148 Did you choose this? 5 x x 148 I can easily double 5. I can take half of x 10 x 74 = 740 Now it’s easy with 10. How did you do?

This method of multiplying supports mental math. This method also supports the properties of multiplication and division. Students feel SO powerful when they can use this! This method of multiplying supports mental math. This method also supports the properties of multiplication and division. Students feel SO powerful when they can use this!