SOLUTION: A boat can travel with a speed of 15 km/hr in still water. it needs 4 more hours to travel 63km agains the current of a river than it needs to travel down the river find the speed

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Question 696737: A boat can travel with a speed of 15 km/hr in still water. it needs 4 more hours to travel 63km agains the current of a river than it needs to travel down the river find the speed of the river
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let +t+ = the boat's time in hrs to travel 63 km downstream
Let +c+ = the speed of the current in km/hr
The boat's speed going downstream is +15+%2B+c+
The boat's speed going upstream is +15+-+c+
------------
given:
Equation for going upstream is
(1) +63+=+%28+15+-+c+%29%2A%28+t+%2B+4+%29+
Equation for going downstream is
(2) +63+=+%28+15+%2B+c+%29%2At+
----------------------
(1) +63+=+15t+-+c%2At+%2B+60+-+4c+
(1) +15t+-+c%2At+=+3+%2B+4c+
(1) +t%2A%28+15+-+c+%29+=+3+%2B+4c+
(1) +t+=+%28+3+%2B+4c+%29+%2F+%28+15+-+c+%29+
----------------------------
Substitute (1) into (2)
(2) +63+=+%28+15+%2B+c+%29%2A%28+3+%2B+4c+%29+%2F+%28+15+-+c+%29++
(2) +63%2A%28+15+-+c+%29+=++%28+15+%2B+c+%29%2A%28+3+%2B+4c+%29+
(2) +945+-+63c+=+45+%2B+3c+%2B+60c+-+4c%5E2+
(2) +4c%5E2+-+126c+%2B+900+=+0+
(2) +2c%5E2+-+63c+%2B+450+=+0+
Use quadratic equation:
+c+=+%28-b+%2B-+sqrt%28+b%5E2-4%2Aa%2Ac+%29%29%2F%282%2Aa%29+
+a+=+2+
+b+=+-63+
+c+=+450+
+c+=+%28-%28-63%29+%2B-+sqrt%28+%28-63%29%5E2-4%2A2%2A450+%29%29%2F%282%2A2%29+
+c+=+%28+63+%2B-+sqrt%28+3969+-+3600+%29%29%2F4+
+c+=+%28+63+%2B-+sqrt%28+369+%29%29%2F4+
+c+=+%28+63+%2B-+sqrt%28+369+%29%29%2F4+
You'll have to finish, but this is the basic idea
of how to solve. Then check the work.