# Ratios & Percentages

### Topics

## Introduction to :

Most of the time in life, you will estimate percents. You probably won’t bust out a pen and paper every time you need to pay a tip or figure out the discount on a shirt. If you get good at *estimating* percents, you’ll get close enough.

The first thing to do is round the amount to a nice easy number to work with.

## Shortcuts for Estimating Percentages

% | Trick | Round Amount | Estimate |
---|---|---|---|

10% | Move the decimal one place to the left | $72.38 -> $70.00 $58.20 -> $60.00 $26.00 -> $25.00 |
10% of $70.00 is $7.00 10% of $60.00 is $6.00 10% of $25.00 is $2.50 |

20% | Double 10% | $72.38 -> $70.00 $58.20 -> $60.00 $26.00 -> $25.00 |
20% of $75.00 is $15.00 20% of $60.00 is $12.00 20% of $25.00 is $5.00 |

25% | Round amount to a number divisible by 4, then divide the amount in fourths | $72.38 -> $72.00 $58.20 -> $60.00 $26.00 -> $28.00 | 25% of $72.00 is $18.00 25% of $60.00 is $15.00 25% of $28.00 is $7.00 |

50% | Cut the amount in half | $72.38 -> $72.00 $58.20 -> $60.00 $28.00 -> $28.00 | 50% of $72.00 is $36.00 50% of $60.00 is $30.00 50% of $28.00 is $14.00 |

Do the following problems in your head. Feel free to round the numbers.

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