Åk 6–9
 
English/Русский
5.4 Setting up multiplication
When using the multiplication method (multiplication algorithm), we often use both multiplication and addition. Here you need to keep track of carrying over numbers and which number belongs to the ones, tens, hundreds and so forth.

Make sure that you know your multiplication tables – it makes it much easier! Don’t forget to estimate if your answer seems reasonable.



Example A: 243 ∙ 2 = 486

Set this up so that the numbers with the least number of digits is at the bottom.  You usually do so in order to make the calculations easier.


Multiply 2 with the ones:
2 · 3 = 6


Multiply 2 with the tens:
2 · 40 = 80


Multiply 2 with the hundreds:
2 · 200 = 400


The answer becomes four hundred eighty six.


Example B: 26 ∙ 254 = 6,604

Set this up so that the factor with the most digits is at the top.


We begin by multiplying the ones with 254:
6
∙ 254
Look at example A and do the same.




Multiply then the tens with 254: 20 254

It is very important that you begin with the tens digit from the right to the left when you write your answer because it is the tens digit you multiply with.

 



Here you then add both rows with the usual addition method.

The answer becomes six thousand six hundred four.

What we have done here is to first multiply 6 by 254 and then 20 by 254 and then add the sums together.  This then becomes work in addition.

26 · 254 =(20 + 6) · 254 = (6 · 254) + (20 · 254) = 1,524 + 5,080 = 6,604


Example C: 0.13 ∙ 0.6 = 0.078

Set up the numbers to that the factor with the most digits is on top.


Multiply 6 tenths by 3 hundredths.
0.6 · 0.03 = 0.018
The one will then be carried.
Write down the 8.


Multiply 6 tenths by 1 tenth.  
0.6 · 0.1 = 0.06
Carry the 1:
0.06 + 0.01 = 0.07
Write down the 7.


Because all multiplications left will give a product of 0, we are finished and it is time to count the number of decimals.

The top factor contains 2 decimals and the lower factor contains one.  So there is a total of three decimals in the answer.

The answer become seventy eight thousandths.