Question

Erica and Amy started biking in the opposite directions at the same time from the same place. Erica biked at a speed of 6 meters per second and Amy biked at a speed of 4 meters per second. How soon were they exactly 1 km apart?

Answer 1

Since they are moving in opposite directions, this is the same as Amy staying still and Erica biking at 10 meters per second away from her.

To travel 1km = 1000m at 10 meters per second, you need 1000/10 = 100 seconds.