Dont understand

Hi Ella, yes, for binary group games (in/out) games where all the variables have to be either in one group or the other and are not repeated we can write contrapositives slightly differently. Take rule one for example. K/r ->J/s Our "traditional" contrapositive would be Not J/s -> Not K/r However, since we know when J isn't in S it's in R, and when K isn't in R it's in S, we can write this contrapositive as J/r -> K/s (logically identical to Not J/s -> Not K/r). Note that if you are doing a grouping game with two groups and either not all of the variables are used or some variables are repeated then you should stick to the traditional method of Not J/s -> Not K/r However, in most binary group games, writing the other version is generally quicker and easier to make inferences with. Feel free to follow up if you have further questions!