-A 180 kg hippo is riding a bicycle at a speed of 6.0 m/s. Out of nowhere, a chimpanzee runs out in the
hippo's path and he must slam on his brakes. If the
bicycle's brakes exert a force of 1600 N, how
long will it take to stop the bicycle? _s

It is either a  proton or a neutron im almost positive. hope this .

Distance covered = speed x timethe distance traveled going against the stream = distance traveled with the stream.therefore it is the speed and the time that are the variables.as he travels the same distance both ways, speed x time is the same for each way.going upstream his speed is 4 kph; going downstream his speed is 6 kph.therefore we have two simultaneous equations: time upstream x 4 = time downstream x 6time upstream + time downstream = 60 minutestherefore we have the equation: time upstream = 6/10 hour, time downstream therefore = 4/10 hour.distance traveled at 4km/hr in 6/10 hour = 24/10 = 2.4km.check: distance traveled at 6km/hr in 4/10 hour = 24/10 = 2.4km.