# Number Puzzle #02- Flat Tire -

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?

**HINT**: Time = Distance / Speed.

**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.)?