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Mr. Wiley averages 12 mph when his boat is in still water. One day he goes 12 miles upstream and the same distance back. The round-trip takes two hours and 15 minutes. What is the speed of the current on that day?

Sagot :

Sqdancefanidn

Answer:

  4 mph

Step-by-step explanation:

The current subtracts from the boat speed going upstream, and add going downstream. The relation between speed, distance, and time is ...

   time = distance/speed

  2 1/4 = 12/(12 -c) + 12/(12 +c) . . . . . round trip time is the sum of the trip times

  9/4(12 +c)(12 -c) = 12(12 +c) +12(12 -c) . . . . multiply by (12 +c)(12 -c)

  144 -c^2 = 128 . . . . . . multiply by 4/9 and simplify

  16 = c^2 . . . . . . . . . add c^2 -128

  c = 4 . . . . . . . . . positive square root

The current was 4 mph that day.

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