**How to Do Percentages**

This note was sent to
the HSTUAC home school e-mail discussion list in response to a mother's concern
about her son. He didn't like to
"follow the rules" in doing mathematics. Instead, he liked to find his own way of doing things.

This may not be
bad. I have no problem with letting him
use his own method as long as the method is valid. I'll give you an example.
At one point, if my son was asked to find 32% of 150 he would say 10% is
15 and 1% is 1.5. So 30% is 45 and 2%
is 3. So 32% is 45 + 3 or 48. Perfectly valid. Not the most efficient way but it works. Of course, if it was 35.68% of 438.976 his method
would be a real pain but it is still correct.

Now, if he is reducing
the fraction 16/64 to 1/4 by "canceling" the 6's, though the answer
is right the method is bogus.

So just look at how he
does it. If it's legitimate, don't
stress too much about it. Show him the
other way but don't be too concerned.
I've been showing my son the other way to do percentages and I think he
finally sees the advantage but I was pleased that he understood what was going
on.

The way I teach people
to do percentages is the following:

Every percentage problem
can be looked at as an equation with two fractions.

part PART

--------- =
------------

whole WHOLE

For example, what is 32%
of 150?

Per cents are fractions
over 100 so 32 is a part and 100 is a whole.

32 PART

------ =
------------

100 WHOLE

The wording of the
problem shows that 150 is the "WHOLE" and the "PART" is unknown. So, we have:

32 x

------ = --------

100 150

So

32 (150)

------------- = x

100

or x = 48.

Doing it this way helps
them to see what's really going on and it keeps them from having to memorize
how to do 3 different kinds of problems.
Instead of remembering how to do "32 is what percent of 98?"
and "32 is 47% of what number?" and "what number is 45% of
68?" all they have to remember is

part PART

--------- =
------------

whole WHOLE