Atoms of element a decay to atoms of element b with a half-life of 20,000 years. if there are 10,000 atoms of a to begin with (and 0 atoms of b), how long will it take for there to be 2,500 atoms of a?

when two balls of the different masses are dropped from equal height, they reach the ground at the same time .this is because they experience the same acceleration.

the acceleration is independent of mass.

thus when they are dropped they will always maintain the same velocity and travel the same distance (neglect air resistance of course) at the same time.