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Make that 10 hard problems.

Similar to - There are 10 kinds on people. Those who understand binary and those who don't.

Every base is base 10.

I found this one particularly amusing :)

It took me a while to notice the aliens hands...

My favorite take on that one is more for compiler and embedded folks:

There are 1 kinds of people in this world. Those who know how to save bits. The rest are the default case.