Need help getting through this after part a, my work is here so you can go off of it thank you

This is the trickiest part of the question, so let's break it down. Suppose there are f floating variables and d dangerous sets. (i) Each floating variable can be in at most t dangerous sets. So argue that if we sum up the number of floating variables in each dangerous set, we get at most ft. (ii) Each dangerous set contains at least t + 1 floating variables. So if we again sum up the number of floating variables in each dangerous set, we get at least d(t+1). (iii) Write the inequality that relates ft and d(t+1) and conclude that d