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A red car and a blue car are traveling at the same speed. The red car drives 4 hours. The blue car drives an additional 45 minutes and goes 21 more miles than the red car. How fast are the cars going?

 Feb 12, 2021
 #1
avatar+120129 
+1

Let  the speed  of  the two cars  be S   (mph)

 

So......the distance  the red car  goes in 4 hours  + 21 miles = the distance  the blue car goes in 4 + 3/4  hours  = 4.75 hours

 

So

 

4S  +  21  =  4.75 S        subtract  4S from each side

 

21  =  .75S       divide both sides  by  .75

 

S   =  28  (mph)

 

Another  way to see this is that the blue car  covers 21  miles in 3/4 hr

So   21/ (3/4)  =   84 / 3  = 28 mph

 

cool cool cool

 Feb 12, 2021
 #2
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+1

It takes blue car 45 min to go 21 miles

21 m  / (3/4 hr) =   28 m/hr

 Feb 12, 2021

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