Question

We are using Chi Squares to understand if descrepancies in aniaml populations are random or significant. On the developed land, 20 traps were set. 7 Road runners were caught, and two were marked. On the Undeveloepd land, 20 traps were also set, but 11 Road Runners were caught and 1 was marked. I know the formula for the chi square, x^2=sigma (observed-expected)^2/expected. But which values are observed, and which are expected? And how do the marked values fit into this?