a boat moves upstream at the rate of 1 km in 20 minutes and down stream 1 km in 15 minutes . then the speed of the current is :
"rate upstream = ( 1 / 20 * 60 ) = 3 kmph rate down stream = 1 / 15 * 60 = 4 kmph rate of the current = ½ ( 4 - 3 ) = 0.5 kmph answer : b"
divide(subtract(multiply(divide(1, 15), const_60), multiply(divide(1, 20), const_60)), const_2)