X+Y=100 In this case X would be Dogs and Y would be Ravens. We are told that X (dogs)+ Y (ravens) = 100. Each dog has 4 legs. 4x?=140. Each raven has 2 legs. 2x?=130. 140+130=270. ?=?=100. 35+65=100. 4x35=140, 2x65=130. 140+130=270 and 35+65=100. So we know that there are 35 Dogs and 65 Ravens making 270 legs all together and 100 heads.
