Via Jonathon Scwharz at A Tiny Revolution, we have this... ...with the comment, "If you figure out how old the father and son are in this cartoon (found here), it actually shapeshifts from something depressingly unfunny to a small masterpiece of alarming creepiness." The link in the quote will take you to the source of the above, and the link above the comic will take you to the answer. I actually took a little bit of pleasure from dusting off my badly unused algebra skills and working this out for myself; it went quickly. Sad to say, this is the kind of problem I used to solve in my head in a few seconds. Now I have to write it all down to make sure I'm keeping everything straight. As for the first picture, I grew up with "New Math" and that sort of problem was what we were doing in first grade, albeit with an empty box in place of "X." So by the time I got to "actual" algebra in 9th grade, it seemed second nature.
Hint as to why the answer is sort of deliciously creepy: think Michael Douglas and his more recent progeny in 10-15 years.
Followup: As long as we're on the subject of stupid math tricks, today is international palindrome day, 01022010. Via Pygalgia (where I saw it first), among others.