Question

David rowed a boat upstream for three miles and then returned to point he started from.

The entire journey took four hours.
The speed of the stream is one mile per hour.
Find David's speed in still water.
David's speed in still water is ? miles per hour?

Answer 1

The speed of the stream: v s = 1 mph
v - David`s speed in still water
( v - 1 ) t 1 = 3
( v + 1 ) t 2 = 3
t 1 + t 2 = 4
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t 2 = 4 - t 1
3 = ( v - 1 ) * t 1
3 = ( v + 1 ) * ( 4 - t 1 )
-----------------------------
3 = v t 1 - t 1
+
3 = 4 v - v t 1 + 4 - t 1
------------------------------
6 = 4 v + 4 - 2 t 1
2 = 4 v - 2 t 1
2 t 1 = 4 v - 2 / : 2 ( divide both sides of equation by 2)
t 1 = 2 v - 1
t 2 = 5 - 2 v
------------------
3 = ( v - 1 ) * ( 2 v - 1 )
3 = 2 v² - 3 v + 1
2 v² - 3 v - 2 = 0
v 1/2 = 3+/- √(9 + 16 ) / 4 = (3 + 5)/4 = 8/4 = 2 ( other solution is negative )
Answer: David`s speed in still water is 2 miles per hour.