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 25.
32 x 50
I can easily take half of 32 and double 50.
16 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 55
45 x 36
5 x 148
Did you choose this?
5 x 148
5 x 148
I can easily double 5.
I can take half of 148.
10 x 74
Now it’s easy with 10.
10 x 74 = 740
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!