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katy bicycle distance problem

katy left her house on a bicycle heading north at 8mph.  At the same time her sister molly headed south at 12 mph.  How long will it take for them to be 24 miles apart?
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1 Answer

Hi again Amie;
Katie, 8 m/h North
Molly, 12 m/h South
 
They are traveling away from each other at a rate of (8+12) m/h.
distance=(rate)(time)
(distance)/(rate)=time
(24 miles)/(20 miles/hour)=time
Let's cancel miles located in both the denominator in numerator...
(24 miles)/(20 miles/hour)=time
Let's note the fact that the unit of hour is a denominator within the denominator.  Henceforth, it is the numerator.
[(24)/(20)]hours=time
1.2 hours=time
Perhaps it would be better to express this in hours and minutes?
(1 hour)+(0.2 hours)=time
(1 hour)+[(0.2 hours)(60 minutes/1 hour)]=time
Let's cancel units where appropriate...
(1 hour)+[(0.2 hours)(60 minutes/1 hour)]=time
(1 hour)+(12 minutes)=time
 
NOW GET SOME SLEEP!