Two trains on opposite tracks leave the same station at the same time. one train travels at an average speed of 80 km/hr and the other travels at an average speed of 70 km/hr. how long after they leave the station will they be 50 km apart?

Johnny rode his bike to a friends house 4 blocks down the street in his neighborhood. he immediately rode back home once he realized his friend was unable to play. what was his displacement for the total bike ride trip? what could you use as a reference point ? show the steps to solve this problem.