A boat crossed a lake from North to South at the speed of 4 km/h, entered a river and covered twice 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 3.5 km/h that day, what was its approximate downstream speed? Choose the correct option.
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 5D is the total distance, then distance covered in lake = D, distance covered upstream = 2D and distance covered downstream = 2D   =&gt;5D/3.5 = D/4 + 2D/3 + 2D/x (where x is the downstream speed)   =&gt; 5/3.5 = 1/4 + 2/3 +2/x (div. both sides by D)   =&gt; 1.42 = .25+.66 +.51/x =&gt; x=3.9 ~ 4   option 4
<result>A</result>
A boat crossed a lake from North to South at the speed of 4 km/h, entered a river and covered twice 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 3.5 km/h that day, what was its approximate downstream speed?