The most reasonable way to go about figuring out the answer is to make a
list of
possibilities. For instance, you can buy 6 McNuggets (of course), but you
can't
buy 7 or 8. You can buy 9, but not 10 or 11. A little farther along in
the list, you will find that you can buy 24 McNuggets (24=9+9+6) but not
25,
and then you can buy 26 (=20+6) And so forth.
Before you go too far, think about how you will know when you've gone
far enough. The answer to this problem is larger than you might think at
first!
Another strategy might be to do a simpler problem first -- try a
situation
where you can by 5 or 8 McNuggets, for example.
As always, be sure to explain the
work you did to arrive
at your answers!
This is an unusual question, because it's about impossibility
rather than
possibility.