We see them all the times, some companies are still using them to hire people - a bad idea in my opinion. Anyway I've found this one posted on Facebook by one of my friends, and supposedly it went viral and was seen more than a million times. I've solved it in 10 seconds. 

Can you do better? In any case, I think that an even better skill to have (in job interviews) is to think about doing a Google search for "15 + 3 = 1218" or "6 + 4 = 210". That's what I did, and that's how I found a webpage providing the solution. Though I solved it myself independently before seeing the solution. I think this one is a very easy one.