A cyclist rode his bike for 4 hours. In that time, he rode 70 miles. What was his speed in miles per hour?

0.057 miles per hour
17.5 miles per hour
28 miles per hour

Question

A cyclist rode his bike for 4 hours.  In that time, he rode 70 miles.  What was his speed in miles per hour? 
 
       0.057 miles per hour
        17.5 miles per hour
        28 miles per hour

whpalmer4 · Answer

70 miles divided by 4 hours.  How many miles traveled in each hour, assuming that the speed is constant?

anonymous · Answer

17.5?

whpalmer4 · Answer

Let's check: if you ride for 4 hours at 17.5 miles/hour, how far do you go?

$$d = rt$$$$d = 17.5\frac{\text{ miles}}{\text{ hour}}*4\text{ hours} = 17.5*4 \frac{\text{ miles}}{\cancel{\text{ hour}}}\cancel{\text{ hours}} = 70 \text{ miles}$$Looks good!

anonymous · Answer

thanks :)

whpalmer4 · Answer

You can also quickly see that neither of the other two answers is reasonable.  If we only go 0.057 miles in an hour, we have to travel more than 16 hours just to go the first mile!  In fact, that's only about an inch each second, or about 9 times the speed of the fastest garden snail.  

If we try 28 miles per hour, 4*28 is about the same as 4*30, so we're looking at well over 100 miles traveled.  Clearly not the right answer, either.