Turn difficult multiplications into easier ones
Halve one number and double the other. The numbers change, but the answer stays the same.
Take 16 × 5. Multiplying by 5 can take a moment, but multiplying by 10 is often immediate. Halve 16 to get 8, double 5 to get 10, and solve 8 × 10 = 80. This works because you made half as many groups, with twice as much in every group.
The same 24 dots arranged in an easier way
Six rows of 4 dots and three rows of 8 dots both contain 24 dots. Halving the number of rows and doubling the dots in each row does not change the total.
6 × 4=3 × 8= 24
How to calculate 16 × 5 in your head
Change the multiplication into 8 × 10, which is easier to solve.
-
Halve 16
16 ÷ 2 = 8
You now have half as many groups.
-
Double 5
5 × 2 = 10
Put twice as much in each group, so the total does not change.
-
Solve the easier multiplication
8 × 10 = 80
Eight groups of 10 contain the same amount as 16 groups of 5.
More multiplications you can simplify
Start by halving one number and doubling the other. When neither number halves cleanly, you can sometimes use a separate fallback: double one factor, solve, then halve the result.
14 × 5
= 70
7 × 5
= 35
16 × 25
= 400
When doubling and halving helps
Use the method when the new multiplication is easier for you than the original one.
Try it when
- One number is even, so you can halve it without creating a fraction.
- Doubling the other number creates a familiar number such as 10, 20, 50, or 100.
- You can repeat the change if the halved number is still even.
Choose another method when
- Both numbers are odd and doubling one of them does not create an easier multiplication.
- The new numbers are no easier to multiply than the original ones.
Now try 18 × 5
18 × 5 = ?
Halve 18 and double 5. What easier multiplication do you get?
Show the answer Hide the answer
Half of 18 is 9, and double 5 is 10. So 18 × 5 becomes 9 × 10 = 90.
Where this can save you time
The method helps when quantities come in equal groups and one number can be halved cleanly.
Calculate a price
Eight items at $2.50 cost the same as four pairs at $5.00: 8 × $2.50 becomes 4 × $5.00 = $20.
Count equal rows
Sixteen rows with 5 seats contain the same number of seats as eight rows with 10: 16 × 5 becomes 8 × 10 = 80.
Try doubling and halving while you play
Look for multiplication by 5, 25, or 50. Halve the other number and double 5, 25, or 50 to create an easier multiplication.
Questions about doubling and halving
Why does doubling and halving keep the answer the same?
You make half as many groups, but place twice as much in each group. Those two changes cancel each other, so the total stays the same.
Can I double and halve more than once?
Yes. For 16 × 25, use 8 × 50 and then 4 × 100. Each step keeps the same answer while making the multiplication easier.
Can I use this when both numbers are odd?
Sometimes. Instead of halving a number first, double one number, solve the easier multiplication, then halve the answer. For 7 × 5, calculate 7 × 10 = 70, then halve 70 to get 35.