SOLUTION: It takes a boat going upstream 11 hrs to cover the same distance, as it would cover in 9hrs going downstream. What is the speed of the boat if the speed of the current is 2km/hr?

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Question 980898: It takes a boat going upstream 11 hrs to cover the same distance, as it would cover in 9hrs going downstream. What is the speed of the boat if the speed of the current is 2km/hr?
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let +d+ = the one-way distance the boat travels in km
Let +s+ = the speed of the boat in still water in km/hr
+s+%2B+2+ = the speed of the boat going downstream in km/hr
+s+-+2+ = the speed of the boat going upstream in km/hr
------------------------------------------------------
Equation for going upstream:
(1) +d+=+%28+s-2+%29%2A11+
Equation for going downstream:
(2) +d+=+%28+s%2B2+%29%2A9+
-------------------
(1) +d+=+11s+-+22+
and
(2) +d+=+9s+%2B+18+
--------------------
By substitution:
+11s+-+22+=+9s+%2B+18+
+2s+=+40+
+s+=+20+
The speed of the boat is 20 km/hr
check:
(1) +d+=+%28+s-2+%29%2A11+
(1) +d+=+%28+20-2+%29%2A11+
(1) +d+=+198+ km
and
(2) +d+=+9s+%2B+18+
(2) +d+=+9%2A20+%2B+18+
(2) +d+=+198+ km
OK