The strategy
How it works
Find an even factor that can be halved.
Double the other factor by the same factor.
Multiply the new pair and check that the transformation balances.
See it in action
Worked examples
Example 116 × 25
- Halve 16 to 8.
- Double 25 to 50.
- 8 × 50 = 400.
Answer: 400
Example 214 × 5
- Halve 14 to 7.
- Double 5 to 10.
- 7 × 10 = 70.
Answer: 70
A quick heads-up
Common mistakes to avoid
- Doubling or halving both factors; only adjust one each way.
- Halving an odd factor without checking for a fraction; choose a clean even factor.
Your turn
Check your understanding
Choose an answer to see instant feedback and a short explanation. You can take the quiz again any time.
Question 1 of 3
Rewrite 12 × 5 by halving 12 and doubling 5.
Half of 12 is 6 and double 5 is 10; the product remains 60.
What is 18 × 5?
Half 18 to 9 and double 5 to 10. Then 9 × 10 = 90.
Which pair has the same product as 20 × 15?
Halve 20 and double 15: 10 × 30 remains 300.
Keep the momentum
Try a few practice problems
Choose a comfortable range and work through a short round with the free mental math trainer.