The reason why we add 49 plus 36 is because that is the number of children that are wearing a costume out of the original 103. This leaves with a total of 85 children that we know for sure wore some kind of costume.
103 - 85 equals 18.
We are subtracting 103 - 85 because we are taking away the number of students we know for sure that have a costume to find out the number of students who don't have a costume.