## Boats And Streams

A boat moves downstream at the rate of 1 km in $${\text{7}}\frac{1}{2}$$ minutes and upstream at the rate of 5 km an hour. What is the speed of the boat in the still water?
Options:
 A. 8 km/hour B. $${\text{6}}\frac{1}{2}$$ km/hour C. 4 km/hour D. $${\text{3}}\frac{1}{2}$$ km/hour