A farmer has some (four-legged) sheep and some (two-legged) chickens. There are 30 animals in total. The total number of legs for these 30 animals is 84. How many animals of each kind are there?

This type of problem often frustrates people who dislike math, but it is really pretty simple. Start by pretending that the sheep are all standing on two legs instead of four. How many legs would the 30 animals have? Subtract this number from the 84 legs that the problem calls for. Every sheep is going to add 2 to the leg count. You are almost there.