Question 351783
A motorboat can travel upstream on a river at 12 mph and downstream at 20 mph. 
How far upstream can the boat travel if it leaves at 8:00 am and must return by noon?
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From the information given we know we have to complete the round trip in 4 hrs
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Let d = one way distance
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Write a time equation: time = dist/speed
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Time up + time down = 4 hrs
{{{d/12}}} + {{{d/20}}} = 4
Clear the denominators multiply by 60
60*{{{d/12}}} + 60*{{{d/20}}} = 60(4)
Cancel the denominators and you have:
5d + 3d = 240
8d = 240
d = {{{240/8}}}
d = 30 miles up-stream to return at noon
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Confirm this, find the times
{{{30/12}}} + {{{30/20}}} = 
2.5 + 1.5 = 4hrs
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That was pretty easy, right?