An old car travelling from Los Angeles to San Diego cruises at a speed of 21 miles per hour. After
reaching San Diego, the car immediately does a u-turn and returns to Los Angeles at a speed of only
3 miles per hour (courtesy of a flat tire).

What is the car's average speed over the entire journey?

**ANSWER**:

*5.25 miles per hour.*
**EXPLANATION**: Let's use

**x** to signify the distance between Los Angeles and San Diego. Since time
traveled (hours) equals distance traveled (miles) divided by average speed (miles per hour), it takes

**x/21** hours to travel to San Diego and then

**x/3** hours to return to Los
Angeles. The total time on the road is therefore

**x/21** +

**x/3**. To add
these fractions, we need to use a common denominator of 21:

**x/21** +

**7x/21**
=

**8x/21** hours. So it takes

**8x/21** hours for the return trip of

**2x** miles or more simply

**4/21** hours to travel 1 mile. Therefore, the
average speed is

**21/4** or 5.25 miles per hour.

Do you have a

suggestion for this puzzle (e.g. something that should
be mentioned/clarified in the question or solution, bug, typo, etc.)?