A boat crossed a lake from North to South at the speed of 6 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 5 km/h that day, what was its approximate downstream speed?
Choose the correct choice from the following choices:
A) 12
B) 15
C) 16
D) 17
E) 18
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/5 = D/4 + 2D/3 + 2D/x (where x is the downstream speed)   =&gt; 5/5= 
<gadget id="calculator">5/5</gadget>
<output>1</output>
1/6 + 2/3 +2/x (div. both sides by D)=&gt; 1.0 = .16+.66 +.16/x =&gt; x=11.98 ~12   option 12
<result>A</result>
A boat crossed a lake from North to South at the speed of 6 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 5 km/h that day, what was its approximate downstream speed?