Question 203796: A girl walked 1/3 of the way at 3 mph and got a car ride the rest of the way at 30 mph. What was her average speed?
Found 2 solutions by stanbon, MathTherapy:
Answer by stanbon(75887)   (Show Source):
You can put this solution on YOUR website! A girl walked 1/3 of the way at 3 mph and got a car ride the rest of the way at 30 mph. What was her average speed?
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average speed = total distance/total time
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Your Problem:
Let the total trip distance be x miles:
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Walking time = distance/rate = [(1/3)x]/[3 mph] = x hrs
Riding time = distance/rate = [(2/3)x]/[30 mph] = 20x hrs.
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Therefore average speed = x miles/(21x hrs) = (1/21) mph
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Cheers,
Stan H.
Answer by MathTherapy(10552)  (Show Source):
You can put this solution on YOUR website!
D = RT
Let the distance the girl traveled be d. Since her rate of speed is 3 mph, then for the first  of the journey, her time to cover “d” distance is:
 , or  , or 
Since her rate of speed for the rest of the journey,  is 30 mph, then for the rest of the journey, her time to cover that distance is:
 , or  , or,  . The total time she took for the entire journey is therefore  , or  = 
Since her total time to cover the entire distance is , and since her total journey = “d,” then her average speed =  , or,  =  mph.
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