THE BEST PUZZLES #2 – “Three Nines”

I have collected (and made) a lot of cool maths & logic puzzles over the years.
As too many Facebook maths problems today seem uninspired, I have decided to start posting from my collection.

Select the text in yellow line below to see the answer.

This particular puzzle is from a great book that I got for free when a local library discarded it.
Teaching Middle School Mathematics [Brumbaugh, Ortiz & Gresham]