Multiplying by 20, 25, and 30

Multiplying By Twenty

Multiplying by 20 is done in two stages, by 2 and then by 10.


Multiplying By Twenty Five

To multiplying by 25:

Divide the number by 4. Then:
If you have a remainder of 0 then add 00 at the end.
If you have a remainder of 1 then add 25 at the end.
If you have a remainder of 2 then add 50 at the end.
If you have a remainder of 3 then add 75 at the end.

Example: 143 * 25

143 / 4 = 35 remainder: 3

so
143 * 25 = 35 & 75 (for a remainder of 3 add 75 at the end)
143 * 25 = 3575


Multiplying By Thirty

Multiplying by 30 is done in two stages, by 3 and then by 10.

Multiplying by 5, 10, and 15

Multiplying By Five

Multiplying by 5 is just multiplying by 10 and then dividing by 2. Example:

15 * 5 = (15 * 10) / 2 = 150 / 2 = 75


Multiplying By Ten

Everybody knows that to multiply by 10, a zero should be added to the end of the number. Example:

26 * 10 = 260


Multiplying By Fifteen

To multiply by 15, you can use one of these two methods:

1) Multiply by 5 then then add it up with the product of the number and 10. Example:

43 * 15 = (43 * 5) + (43 * 10) = (43 * 10 / 2) + 430 = 215 + 430 = 645

In other words, add half of the product of the number and 10 to the product of the number and 10; i.e.:

43 * 15 = 430 + (1/2 * 430) = 645

2) Multiply by 30 and then divide by 2 (in the next post, a method is presented for "Multiply by 30"). Example:

57 * 15 = (57 * 30) / 2 = 1710 / 2 = 855

Multiplying by 9, 99, 999, ...

Multiplying by 9 is as multiplying by 10-1. So, 9 * 8 is as (10 - 1) * 8 that is:

9 * 8 = (10 - 1) * 8 = 80 - 8 = 72

Another example:

26 * 9 = 26 * (10 - 1) = 260 - 26 = 234

To multiply by 99, one must multiply by 100-1:

53 * 99 = 53 * (100 - 1) = 5300 - 53 = 5200 + 100 - 53 = 5200 + 47 = 5247

Similarly, to multiply by 999, one must multiply by 1000-1:

87 * 999 = 87 * (1000 - 1) = 87000 - 87 = 86000 + 1000 - 87 = 86000 + 913 = 86913

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