CBEST Math Test Tips - Page five.

Mental Math Tips

Percent means "per hundred." Relate it to dollars and cents. One cent is written as .01. It is equal to 1% of a dollar. Fifty cents is written as .50. It is equal to 50% of a dollar. Remember when writing percents as a decimal, you need a zero in front of a percent that is less than 10.

Example: 2% = .02, 9 % = .09, etc.
If you remember that .50 = 50%, then 5% obviously has to be written a different way.

Know how percentages relate to tenths, twentieths, twenty fifths, and fiftieths, and hundredths.
Examples:

3/10 = 30%
7/10 = 70%
*Note: Each 1/10th =10%
 
3/20 = 15%
11/20 = 55%

*Note: Each 1/20th = 5%

8/25 = 32%
13/25 = 52%
7/25 = 28%
*Note: Each 1/25 = 4%. Just multiply the top by 4 and you have the percentage.

15/50 = 30%
27/50 = 54%
*Note: Each 1/50th = 2%. Double the top number and you have the percentage. See how this is related to 1/25th ?

13/100 = 13%
28/100 = 28%
*Note: Anything out of a hundred is simply that as a percentage.

Know how to rapidly calculate 1%, 5%, 10%, and 50%.

Here's how to calculate 10% rapidly. Move the decimal one place to the left.
Examples:
10% of 75 = 7.5
10% of 365 = 36.5
10% of 1234 = 123.4

5% is simply half of 10%. Divide the value of 10% by 2 and you have 5%. Remember that you should be very good at cutting things in half?
Examples:
5% of 567 = 56.7 ÷ 2 = (50 ÷ 2) + ( 6 ÷ 2 ) + (.7 ÷ 2) = 25 + 3 + .35 = 28.35

Try finding 5% of 673.
Practice this skill!

Here's how to calculate 1%. Move the decimal two places to the left, or one more place after finding 10%.
Examples:
1% of 56 = .56
1% of 145 = 1.45
1% of  78 = .78

If you need something like 7%, find 5% and add 1% + 1%, OR find 10% - 1% - 1% -1%
Example:
Find 8% of 45.
10% - 4.5, so we know the answer is less than 4.5.
1% is .45, so 2% = .90.
Therefore, 8% of 45 = 4.5 - .9 = 3.6
See how easy that was?

Find 9% of 345.
10% = 34.5, 1% = 3.45, so 9% = 34.5 - 3.45 = 31.05

On multiple choice tests, you may not need an exact answer.

This method works for whatever percentage you are asking for.

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