He knew you could reverse 33 groups of 3 penguins into 3 groups if 33. From there he added the 33's together to make 99. He has one penguin left over so this combination can't work.
Math reasoning is a huge component of our math curriculum. My goal in grade two is to get them to reason why a strategy or answer will or will not work. When this happens it's truly amazing to hear them thinking and being logical as to why it does or doesn't work. This boy clearly has mathematical reasoning for this particular problem. He understood that by switching or reversing the combinations, you could find other combinations.
We want them to be thinking about the reasons why it should work because of what the numbers can and can not do. This particular student isn't doing gobs and gobs of math worksheets, instead they are working in a problem based math classroom where it challenges him to think and reasoning. I love it when this happens, now I just need them all to do this :-).