a boat moves upstream at the rate of 1 km in 20 minutes and down stream 1 km in 9 minutes . then the speed of the current is :
"rate upstream = ( 1 / 20 * 60 ) = 3 kmph rate dowm stream = 1 / 9 * 60 = 6.7 kmph rate of the current = ½ ( 6.7 - 3 ) = 1.85 kmph answer : e"
divide(subtract(multiply(divide(1, 9), const_60), multiply(divide(1, 20), const_60)), const_2)