GED Question from Mark: Changing Fractions to Decimals

Yo. Here’s a good question from Mark about somethin’ that’s real helpful for the GED: mental math. That’s doin’ math in your head. And it makes the GED math test easier and faster.

I used to work in a machine shop. Sometimes I would have to convert fractions to inches like 1/3. I know how to do that, all you have to do is divide 1 by 3. The question is, I always had to use a calculator, and I would like to do that without using a calcualtor. Every time I tried I would’nt get the proper answer. How do you do that without using a calculator.
Mark

Yeah, that’s the key, Mark. Doin’ stuff in your head makes it easier and faster, an’ helps a lot on the GED. Part of it is jus’ knowin’ or rememberin’ some of the fractions you see all the time.

Think of it in terms of dollars and cents to remember the real easy ones:

1/4 (a quarter) = 25 cents or .2

1/2 (half dollar) = 50 cents or .5

3/4 (three quarters) = 75 cents or .75

1/10 (a dime) = 10 cents or .1

Tenths are easy. It’s always gonna be point-whatever-is-on-top:

2/10 = .2

3/10 = .3

4/10 = .4

5/10 = .5

6/10 = .6

7/10 = .7

8/10 = .8

9/10 = .9

Fifths is always point-the-top-number-times-2.

1/5 = .2

2/5 = .4

3/5= .6

4/5 = .8

Then, there’s the thirds. These are pretty easy to remember, but they don’t give even numbers:

1/3 = .3333 (remember the 3 from 1/3)

2/3 = .6667 (remember that 2 x 3 = 6)

Then, there’s eighths.

1/8 = half of a quarter = .125 (you can remember it cuz 12 is kinda like 1/2 and 25 is a quarter)

2/8 = 1/4 = .25

3/8 = 1/8 + 2/8 = .125 + .25 = .375

4/8 = 1/2 = .5

5/8 = 1/2 + 1/8 = .5 + .125 = .625

6/8 = 3/4 = .75

7/8 = 3/4 + 1/8 = .75 + .125 = .875

Then, what about more complicated ones? You can still do ‘em in your head. You gotta divide the bottom number into the top, so like 2 into 1 = .5 … on harder ones, it’s harder to do in your head.

Say you have 11/25 … you gotta divide 25 into 11… Start by adding a zero to the 11. So you got 25 into 110… 25 goes into 100 four times, so you got a 4… and ten left over. Add another zero… and that’s 25 into 100 again… that’s 44. Now, where’s the decimal go? Well, it’s before the first number, .44 … you can check it by thinking, 11/25 is almost a half, and .44 in almost .5. So you’re good.

Another way is to try to make the bottom number 100. So, 11/25 = 44/100. Then you take the top number and move the decimal place 2 points, so 11/25 = .44

Here’s some places to go for some more explanation and practice:

http://www.mathsisfun.com/converting-fractions-decimals.html

http://www.curiousmath.com/index.php?name=News&file=article&sid=77

http://www.learningwave.com/chapters/decimal13/basics/convert.html

http://www.coolmath.com/decimals/04-decimals-converting-fraction-to-decimal.html

For more information about the GED test and GED test preparation, visit The GED Academy at http://www.passged.com.