A boat crossed a lake from North to South at the speed of 4 km/h, entered a river and covered thrice as much distance going upstream at 3 km/h. It then turned around and stopped at the south shore of the lake. If it averaged 3.8 km/h that day, what was its approximate downstream speed?
Answers
A) 4 B) 5 C) 6 D) 7 E) 8
Another way will be:   Total time for the trip = distance covered in Lake/speed in still water +distance covered upstream/upstream speed + distance covered downstream/downstream speed   If 7D is the total distance, then distance covered in lake = D, distance covered upstream = 3D and distance covered downstream = 3D   =&gt;7D/3.8 = D/4 + 3D/3 + 3D/x (where x is the downstream speed)   =&gt; 7/3.8 = 1/4 + 3/3 +3/x (div. both sides by D)   =&gt; 1.84 = .25+1 +-59/x =&gt; x=5.05 ~ 5   option 5
<result>B</result>
A boat crossed a lake from North to South at the speed of 4 km/h, entered a river and covered thrice as much distance going upstream at 3 km/h. It then turned around and stopped at the south shore of the lake. If it averaged 3.8 km/h that day, what was its approximate downstream speed?