Hi, yes, this is a sort of before and after question. Now the two ponds have equal number of guppies AFTER transferring. So we'll need to work backwards this time.

So instead of drawing A transfer to B, we can imagine it as B giving back the 218 guppies back to A. So the model of A will increase by 218 (the dotted part of the model), which is the original A (before).

Model of B will decrease by 218, which again will change it back to the original B (before).

I've reduced the model into just one single set of model. Hopefully, looking at the model may help you understand the explanation better.