In a physics experiment, Stan has two lab carts. The blue lab cart has a mass of 0.5 kg and the red cart has a mass of 2.0 kg. In one experiment, Stan set the two carts at rest
on the track. He pushes in the plunger between them and releases it. He uses a
stopwatch to determine that after he releases the plunger, the red cart rolls to the right
at a velocity of 4 m/s.
Determine the velocity of the blue cart after the explosion.

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.

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It is a physical region  hope this : )