It is a bit confusing, but the basics are that 5/5 is 1 or the whole. That means Mark takes the whole of everything to make the sandwiches. If he has two batches and has to make two sandwiches then the whole of everything consists of the two batches, so that means he uses one batch for each sandwich. If he fills the sandwiches he will halve the filling, too. The confusion comes from 5/5 but any number divided by itself is 1 or a whole. 5/5=2/2=1/1 and so on. The number isn't important because the answer is always going to be 1. Actually, there is one exception: 0/0 isn't definable so we always avoid that. But 100/100 is the same as 5/5, Kayla/Kayla is 1!