Multiply by 5, 20 or 25 -

Multiplying and dividing by 10 and by 2 are amongst the easiest of calculations to perform.

Here are some calculations that use a combination of these skills:

Example:

$5 \times 21 = 10 \times 21 \div 2 = 210 \div 2 = 105$

To multiply by 20 is the same as to mutiply by 10 then multiply by 2, or vice versa

Example:

$34 \times 20 = 34 \times 2 \times 10 = 68 \times 10 = 680$

To multiply by 25 is the same as to multiply by 100 then divide by 4, or vice versa

Example:

$24 \times 25 = 24 \div 4 \times 100 = 6 \times 100 = 600$

Here’s an applet to practice with.