Dans poubling and halving can be helpful when carrying out some calculations. You can halve one number and then double the result;
20 x 164 = 10 x 164 x 2 = 1640 x 2 = 3280.
You can partition the number and double part of it and halve its result:
32 x 15 = 32 x 10 + 32 x 10 = 2 = 320 + 320 : 2 = 320 + 160 = 480.
More difficult calcualtions can be made easier by doubling one number in the calculatia
and halving the result: 24 x 15 = 24. x 30 = 2 = 720 = 2 = 360.
To multiply by 50, multiply by 100 and halve the result. To multiply by 25, multiply by 100 a
divide the result by 4.​