Åk 6–9
 
English/Русский
3.4 Multiplying in different ways
It is very useful to be able to have the multiplication table memorized.  Everyone can learn and develop ways to use counting in your head or written help to count in your head.



How would you calculate this exercise in your head?

23 · 4 = ?
13 · 5 = ?

Here are a few suggestions about how you can think when you solve exercises. Think first about how you would do the calculation.


Example 1:

23 · 4 = ?

A. Multiply the tens and ones separately.
23 · 4 = (20 · 4) + (3 · 4) = 80 + 12 = 92

B. Increase and subtract.
23 · 4 = (25 · 4) – (2 · 4) = 100 – 8 = 92

C. Double, double.
23 · 4 = 23 · 2 · 2 = 46 · 2 = 92.


Example 2:

13 · 5 = ?

A. Multiply the tens and the ones for themselves.
13 · 5 = (10 · 5) + (3 · 5) = 50 + 15 = 65

B. Multiply by 10 and halve (divide by 2).
13 · 5 = (13 · 10)/2 = 130/2 = 65