A racetrack measures 9 miles long. If an automobile completes the track in exactly 4 minutes, what is its average rate of speed, in miles per hour?

Question

I am trying to study and I can't quite understand the way to solve the problem because I have not been taught how to solve problems like this yet.

Answer 1

distance = rate x time

 

distance is 9 miles.

 

Time is 4 minutes which is 4/60 = 1/15 hour

 

9 = rate x 1/15

 

15 x 9 = rate  <--- multiplies both sides by 15 to clear the fractions

 

rate of speed is 135 miles per hour

 

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Another way to figure it is 9 miles in 4 minutes.

 

Sets up a ratio:

 

9 miles                ?

----------- =  ----------------

4 minutes        60 minutes 

 

Cross multiplication:

 

9 x 60 = 4 x ?

 

 ?  = 9 x 60 / 4 = 540 / 4 = 270 /2 = 135

 

so 135 miles in 60 minutes=1 hour 

so 135 mph