in one hour , a boat goes 13 km / hr along the stream and 9 km / hr against the stream . the speed of the boat in still water ( in km / hr ) is :
"explanation : let the speed downstream be a km / hr and the speed upstream be b km / hr , then speed in still water = 1 / 2 ( a + b ) km / hr rate of stream = 1 / 2 ( a − b ) km / hr speed in still water = 1 / 2 ( 13 + 9 ) kmph = 11 kmph . answer : option a"
stream_speed(13, 9)