As reemrevnivek explains, the answer to "25% of what?" is the issue here. An exponential decay can be described with the same words.

There's a freaking graph. Are we really being this pedantic that we can't understand what he's saying?

I discovered when I got home that the graph was blocked by my office's web filter. Definitely makes more sense with it there. Thanks for your understanding!

You must be new here!

Sorry, I didn't think of the second interpretation.

So they have a quarter-life of 7 years?