SOLUTION: A boat travels upstream in two rivers A and B. the velocity of the current in river A is 3kph while that of B is 1kph.It takes two times longer to travel 18km in river A than it t

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Question 1081853: A boat travels upstream in two rivers A and B. the velocity of the current in river A is 3kph while that of B is 1kph.It takes two times longer to travel 18km in river A than it took to travel 10km in river B. Calculate the speed of the boat in still water.
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let +s+ = the speed of the boat in still water in km/hr
Let +t+ = time in hrs to travel 10 km in river B
+2t+ = time in hrs to travel 18 km in river A
---------------------------------------------
Equation for going upstream in river A:
(1) +18+=+%28+s+-+3+%29%2A2t+
Equation for going upstream in river B:
(2) +10+=+%28+s+-+1+%29%2At+
----------------------------
(2) +t+=+10%2F%28+s+-+1+%29+
Plug this result into (1)
(1) +18+=+%28+s+-+3+%29%2A2%2A%28+10%2F%28+s+-+1+%29%29+
(1) +18%2A%28+s+-+1+%29+=+20%2A%28+s+-+3+%29+
(1) +18s+-+18+=+20s+-+60+
(1) +2s+=+42+
(1) +s+=+21+
The speed of the boat in still water is 21 km/hr
-------------------
check:
(1) +18+=+%28+s+-+3+%29%2A2t+
(1) +18+=+%28+21+-+3+%29%2A2t+
(1) +18+=+18%2A2t+
(1) +t+=+.5+ hrs
and
(2) +10+=+%28+s+-+1+%29%2At+
(2) +10+=+%28+21+-+1+%29%2At+
(2) +10+=+20t+
(2) +t+=+.5+ hrs
OK