Math Is Fun Forum

John E. Franklin

Member

Registered: 2005-08-29

Posts: 3,588

Learning times tables

If you can try to get your young students
to learn to double numbers in their heads,
then you can teach them to multiply any
number by seven, the hardest number of all.

Say you have 6 times 7.   Take the number
that is not the 7, take the 6 and double it 3
times:   12, 24, 48.  Now simply teach the
kids to try to subtract the 6, not the seven,
from 48 in their heads.   48 - 6 = 42 and
that is the answer.

Now let's try 8 times 7.   This one can be done
2 ways, either by the eights rule, which is
easier than the seven rule, or by the seven
rule.  The eights rule is so easy, you just
double in your head 3 times and your done!!
No subtraction needed at the end.
Using the eights rule, you take the 7 then, not the 8.
Double 7: 14, 28, 56 and you are done 56 is 8 x 7.

Now I'll do it with the sevens rule:
7 x 8.   Start with 8 this time and double 3 times and
subtract an 8 because this is the sevens rule.
double 8:  16, 32, 64, and 64 - 8 is 54 + 2 = 56.
Answer is 56.  Notice the subtract 8 trick.  Subtract 10 and
add 2.

Next I will demonstrate an easy way to multiply by
5 without memorizing the times tables.  Take the other
number and cut it is half, so the student should know
the halfs like 3 1/2 and 4 1/2 and 2 1/2.
For 7 x 5, take the 7 because we are using the 5 rule.
Take half of 7, which is 3  1/2.   Express 3   1/2  in
decimals.  Hopefully they have learned that 1/2 is
point 5.    So 3 1/2 is 3.5 and then move the decimal
over and out of the number to the right, just remove it.
And the 3.5 turns into 35, which is the answer to 7 x 5.

Here is another 5 times problem.
5 x 6.  Take the 6 because this is the 5's rule, not the 5.
Half of 6 is 3, so express this with a decimal.  3.0, then
remove the decimal point and you get 30.

Next I will show you how to teach the children how to
multiply by 6, but to do this they have to have their
3 tables memorized.   Every time you see a 6, just
change it into a 3, but if there are two 6's, just change
one of them.   Like 6 x 6, instead do 3 x 6 and double it.
18 doubled is 36, the answer.
Now do 7 times 6.   Make the 6 a 3.  7 x 3 is 21 and 21
doubled is 42, the answer.
And 9 times 6 is 9x3=27 doubled is 54 the answer.

So now you have rules for timesing by 5, 6, 7, and 8.

The nine's rule is easy, you already know it.
Just subtract one from the number that is not nine, and
make the two digits add up to 9.
For example 9 x 3,  take the 3, not the 9 because this is
the 9's rule, take the 3 and subtract 1:   3-1 is 2.
Now think 2 plus what is 9, it is 7.  So the anwer is 27,
using the 2 and the 7.
One more.  4 x 9.   4-1 = 3     3 and what is 9, that's 6.
So 36 is the answer.

Hope this helps someone out there.  Enjoy teaching...
Good day...   

Last edited by John E. Franklin (2011-03-30 03:15:18)

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