Hey Louis14 , All the best with your final preparations. For this you basically need to understand why you subtract twice in one formula and why add in the other. A - Exactly one group members in A + Exactly 2 Group Members in AB and AC + Exactly 3 group members in ABC B - Exactly one group members in B + Exactly 2 Group Members in AB and BC + Exactly 3 group members in ABC C - Exactly one group members in C + Exactly 2 Group Members in AC and BC + Exactly 3 group members in ABC Thus when you add A+B+C = Exactly one group members individually in A, B and C + 2 (2 group members of AB, BC and AC) + 3 (3 group members of ABC) Finally you want Total = Exactly one group members of A, B and C + Exactly 2 group members of AB, BC and AC + Exactly 3 group members of ABC Thus as you can see when you add A+B+C you have added an extra (Exactly 2 group members of AB, BC and AC) and have added 2 extras of (Exactly 3 group members of ABC) - This is to say when those AB, BC and AC sets given to you do not include 3 group members. Thus you subtract the extras to get the final total. Formula 2 is valid when AB, BC and AC sets include those 3 group members of ABC. If that is the case :- 2 group members of AB and AC; 2 group members of AB and BC; 2 group members of AC and BC will include 3 group members in them as well. Thus when you are subtracting once from A+B+C, you are essentially subtracting 3 the (3 group members). But you need one time the 3 group members of ABC to count them in the total. Hence you add once the 3 group members. I don't how clear was the above explanation, but I hope it does give a lil better understanding.