Multiplying together 2-digit numbers ending in 5

To multiply together 2-digit numbers ending in 5:     ex. 25×45=1125

 

-If the difference in tens digits of the numbers being multiplied is even, the last 2 digits of the answer is 25.      ex.  the difference between 2 and 4 is 2, which is even, giving  _ _25

-Multiply  (lower tens digit) times (1+higher tens digit). To this add half the difference between them. This yields the first digits of the answer.                                                                                           ex.   2 times (4+1) + (half of 2) = 11_ _

 

-If the difference in tens digits of the numbers being multiplied is odd, the last 2 digits of the answer is 75, as in 15×45=675

-Continue the second step above multiplying (lower tens digit) times (1+higher tens digit). Add half the difference between them, as above, throwing away the fraction for the first digits of the answer.          1 times (4+1) + half of 3 = 6_ _      The fraction is thrown away.

Try:     15×55=825      15×65=975       75×86=6375      35×65=2275       45×95=4275

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s