Doubling and Halving

  1. Halve one number (preferably the even one)
  2. Double the other number
  3. Multiply the new pair

Example:

16 × 25
Halve 16 → 8
Double 25 → 50

16 × 25 = 8 × 50 = 400

Another example:

24 × 25
Halve 24 → 12
Double 25 → 50

Halve 12 → 6
Double 50 → 100

24 × 25 = 6 × 100 = 600

Challenge example:

32 × 125
Halve 32 → 16
Double 125 → 250

Halve 16 → 8
Double 250 → 500

32 × 125 = 8 × 500 = 4000

Tip: Use this method when one number is even, the other ends in 5 or 0, and the halving leads to easy multiplication.