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# Expressions with One or More Variables

## Evaluate expressions given values for variables.

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Practice Expressions with One or More Variables

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Are We There Yet?

Imagine that your best friend has moved to a new city. You want to visit her, but you need to know how long the drive will take. You go to an online mapping site and get directions between your town and her town.  How does the computer know how long it will take you to make the trip?

#### An Expression for the Average Traveler

Computer programmers use a basic expression to calculate how long it will take you to get to another city.  They multiply the number of miles you need to travel by the average traveler's speed on that route. They can use the same expression to compute travel times between any two places in the world. All they need to know is the distance and the average speed of travel. Sometimes, a website will offer you two different ways to reach another city. The shorter route may have a longer travel time because you can drive faster on the interstate than you can on the country roads that cut through many small towns. The shortest route isn't always the fastest route.

Sometimes, you'll arrive at your destination earlier or later than the online map predicted. That's because the expression only works for the "average" traveler.  If you stop a lot or get caught in traffic, your trip may be much longer than you expected.  If you drive very quickly or don't take any breaks, you may reach your destination sooner than the computer predicted.  If you take a lot of trips, compare your travel times to the computer's predictions. You'll then be able to see whether the predicted times are generally accurate for you.

#### Explore More

Why is it hard for Google Maps to give users an accurate travel time?

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