MathChat

Math is filled with stuff you need to remember that’s hard to remember, right? Of course right.
So whenever we find a trick for math that’s helpful AND easy, we should learn it, right? Absolutely!

In that spirit, I’ld like to present a trick for converting decimals to percents. To learn the trick, you just need to think back to elementary school and remember alphabetical order. So just dust off the alphabet song you sang in kindergarten and join with me as we master the skill of converting decimals to percents.

    Daily Special:  Conversions Over Easy

First, notice that D comes before P in the alphabet.

So write these two letters in that order, D on the left (then some space), and P on the right, like so:

D                                     P

Next write the words, DECIMAL and PERCENT, after their first letters, like this:

D-DECIMAL                                    P-PERCENT

Now suppose you need to convert a decimal, such as .423, to its percent form. Can’t remember which way you move the decimal point? Eon’t sweat it because all you need to do is go back to your alphabetical diagram. Since we’re going from decimal form to percent form, draw an arrow that similarly goes from the word DECIMAL to the word PERCENT, like this:

D-DECIMAL   –––––––––––––––>     P-PERCENT

That arrow tells you which way to move the decimal point; since the arrow points to the right, you move the decimal point 2 places to the right. So you do this:

0.423 becomes 42.3

See how the decimal point “takes a little journey”? First the decimal point slips from between the 0 and the 4 to between the 4 and the 2. Then it sneaks over again, from between the 4 and the 2 to between the 2 and the 3. At this point you have the decimal point in the right spot, but there’s one last thing you need to do. Can you guess what it is … ?

O.K., here’s the answer: you tack a percent symbol (%) to the right of the right-most digit. So you take your 42.3 and put the % symbol to the right of the 3, like this:  42.3%.

What does this tell us? It says that 0.423 is equal to 42.3%.
As you’ve probably noticed, there are certain math rules that work some of the time but not all of the time. When they don’t work, people call those situations “exceptions,” and exceptions can be pretty annoying. But this decimal-to-percent trick works ALL OF THE TIME: no exceptions. So you can say it is 1.00 reliable (meaning, of course, 100% reliable). And that’s great. But before you try your hand at this skill, let’s check out one more example.

Suppose you’re asked to convert 5.1987 to a percent. Again the decimal point moves two places to the right. So first it slips from between the 5 and the 1 to between the 1 and the 9. Then it moseys to the right again, from between the 1 and the 9 to between the 9 and the 8. So at this point you have: 519.87. But don’t forget the important final step. Get out your “mental thumbtack” and tack a percent symbol after the right-most digit, the 7. So now you have the real answer:  519.87%.

And what does this mean? It tells us that 5.1987 is equal to 519.87%.

O.K., your turn.

Convert each decimal to a percent.

a)  85.9
b)  4.162
c)  208.735
d)  0.047
e)  2074.65
f)  0.283
g)  0.00569

The answers will be at the end of my next blog post.