Answer: 20 minutes( 0.3333 hours) after they leave the station will they be 50 km apart, if the Two trains on opposite tracks leave the same station at the same time.
Step-by-step explanation: One train travels at an average speed of 80 km/hr and the other travels at an average speed of 70 km/hr.
There are 6! possible permutations of the blue books, 2 possible permutations of the red books, 4! possible permutations of the green books and 3! possible permutations of the three color groups. therefore the total number of ways of arranging the books is given by: