Describe another real-world example of a perfectly elastic collision.

There are none. An elastic collision is one where there is perfect energy transfer between the things that are colliding, and this just doesn't happen in the real world.
This said, it is sometimes convenient to pretend as if real-world collisions are elastic either to illustrate a point or to simplify the underlying mathematics:
It's common to treat pool balls as having elastic collisions because there's a pretty good transfer of energy between them.

The kinetic molecular theory of gases assumes that any collisions between gas molecules are completely elastic, which is a pretty good approximation for most gases.

Other collisions are not very elastic and never treated as such. For example, if I drive a Buick into a wall, most of the energy goes into squishing the Buick, rather than just transferring it to the wall. If it was a simple energy transfer, the Buick would be just fine afterwards.

The half life of radium is 1600 years after how many years 25% of a radium block will remain Undecayed

25% of Radium block remains undecayed after 3200 years.

Explanation: