Mary and Henry are meeting up for lunch. The total was $18.50. Since Henry helped Mary out earlier in the week, Mary offers to pay for lunch. Henry offers to pay the tip. He’s not sure how much tip to leave, but he knows he wants to leave around 15%. How can Henry estimate 15% of $18.50?

In this concept, you will learn how to use percents to estimate tips.

### Estimating Tips

A **tip** is a sum of money given to a person for performing a service. The amount of tip depends on the type of service being performed, the quality of the service, and the person giving the tip. Tips are often a percentage of the total cost.

Let’s look at an example.

John ate out for lunch. His total bill came to $12.00. He wants to leave a 15% tip.

John can take the time to calculate 15% of the total bill or he can also use what he knows about percents to estimate the approximate tip. 15% is between 10% and 20%.

First, calculate 10% of $12.00. 10% is also 0.1 or one-tenth. When a number is multiplied by one-tenth, move the decimal point one place to the left.

Next, use mental math again to find 20% of $12.00 by multiplying $1.20 by 2.

Then, estimate an amount between $1.20 and $2.40 that will be approximately 15%.

John can estimate that a tip between $1.75 and $2.00 is about 15% of the total bill.

### Examples

#### Example 1

Earlier, you were given a problem about Mary and Henry at lunch.

Henry wants to leave a 15% tip on a $18.50 lunch bill. To find the amount, Henry can estimate by finding close percentages and approximating 15%.

First, round up the total to $20.00 to make estimating easier.

Next, find 10% of $20.00.

\begin{align*}10\% \text{ of } \$20.00 = $2.00\end{align*}

Then, multiply that amount by 2 to find 20% of $20.00

Henry estimates that 15% of $18.50 is approximately $3.00.

#### Example 2

Estimate the tip.

Kelly went to lunch. Her total bill was $29.85. About how much would a 15% tip be?

First, Kelly can round up the total to $30.00 to make estimating easier.

Next, use mental math to find 10% of $30.00. Move the decimal point one place to the left.

This time, use mental math to find half of $3.00. Half of $3.00 is also 5% of $30.00.

Lastly, add the 5% to the 10% to find 15%.

Kelly estimates that a 15% tip will be $4.50.

#### Example 3

Estimate a 15% tip on $25.00.

First, find 10% of $25.00.

Next, find half of $2.50 and add it to $2.50

A 15% tip on $25.00 is $3.75.

#### Example 4

Estimate a 15% tip on $18.00.

First, find 10% of $18.00

Next, multiply $1.80 by 2 to find 20%

Then, estimate an amount between $1.80 and $3.60 that will be approximately 15%.

A 15% tip on $18.00 is between $2.50 and $3.00.

#### Example 5

Estimate a 15% tip on $36.50.

First, round up the total to $40.00 to make estimating easier.

Next, find 10% of $40.00.

This time, find half of $4.00. Add the 10% and 5%.

\begin{align*}\text{Half of } \$4.00 = \$2.00\end{align*}

Since you rounded up the bill to $40, 15% of $36.50 is a little less than $6.00.

A 15% tip on $36.50 is between $5.25 and $6.00.

### Review

Estimate the following tips using 15%.

- A tip on $10.00
- A tip on $18.00
- A tip on $25.00
- A tip on $30.00
- A tip on $15.00
- A tip on $47.00
- A tip on $89.00
- A tip on $121.00
- A tip on $135.00
- A tip on $112.50
- A tip on $127.75
- A tip on $135.50
- A tip on $97.78
- A tip on $88.67
- A tip on $99.20

### Review (Answers)

To see the Review answers, open this PDF file and look for section 8.20.