Squaring 15, 25, 35, ..., 95

To square a two digit number ending in 5; i.e. 15, 25, 35, ..., 95, first multiply the tens digit by the next whole number and then affix 25 to it.

Example 1:

To square 15 (15^2):

The tens digit of 15 is 1 and the next whole number of 1 is 2. So:
1 * 2 = 2
Now, we affix 25 to 2. The answer is 225.


Example 2:

To square 35 (35^2):

The tens digit of 35 is 3 and the next whole number of 3 is 4. So:
3 * 4 = 12
Now, we affix 25 to 12. The answer is 1225.


Example 3:

To square 95 (95^2):

The tens digit of 95 is 9 and the next whole number of 9 is 10. So:
9 * 10 = 90
Now, we affix 25 to 90. The answer is 9025.

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.