Study Guide

Honestly, 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.

% | 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 $70.00 is $14.00 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 $28.12->$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.

A lunch bill is $18.50. Estimate a 20% tip.

First let's find 10% of the bill. We move the decimal to the left by one place to get 10% of $18.50, which is $1.85. We can round this to $1.90 and double it to get an estimate of $3.80 for 20%, or if we're feeling extra nice and a little lazy, we could also round it to $2.00 and double it to get a tip of $4.00.

We can download an album for $12.62 right now, but if we wait till the sale tomorrow it will be 25% off. How much will we save if we wait, and more importantly, is it worth it?

We want to know what 25% of $12.62 is. We can estimate the savings by rounding the cost to $12.00 and dividing by 4 to get $3.00. If we wait, we can save about $3.00. But we're heading out of town tonight and might not be around wi-fi tomorrow, so it's definitely worth the $3.00 to download the album tonight and listen to it during our whole vacation.

Our neighbor has promised to pay us $87.00 to clean up his messy, messy yard. If we have a friend help, we'll get it done quicker, but we'll only get 50% of the money. How much would we make?

We can estimate 50% by rounding $87.00 to $90.00 and splitting that in half to get about $45.00. But we might get upset when we get paid less than we thought. No need to start a fight with the neighbor.

When we're estimating percentages of money, it might be best to round down so that we underestimate and are pleasantly surprised when we get more. We could also round $87.00 to $86.00 and split that in half to get $43.00. This way, when our neighbor pays us and includes a little extra for a job well done, we're super happy to get more than we thought. Cha-ching.