2] A boat can be rowed 9 km upstream or 18 km downstream in a period of 3 hours.What is the speed of the boat in still water.
a] 2.5 kmph  b] 4.5 kmph  c] 6 kmph d] 8 kmph

solution
rate downstream = (9/3) = 3 kmph
rate upstream = (18/3) = 6 kmph
speed in still water = (1/2) (3+6) = 4.5 kmph