 Hello friends, let's discuss the following question. It says how many quads can be drawn to 21 points on a circle? So let us interpret this roughly. We have a circle and we have 21 points on a circle. We know that each chord is the join of two points on a circle and we have to find number of all such quads. Now each chord is the join of two points and by the theory of combination we know that our objects from n objects can be selected or combined knowledge will work as the idea. Let us now move on to the solution. Now we know that each chord join of or combination of two points that is each chord is combination of two points. So the number of quads equal to 21 C2 and this is equal to 21 factorial upon 2 factorial into 21 minus 2 factorial which is again equal to, now 21 factorial can be written as 21 into 20 into 19 factorial upon 2 factorial into 19 factorial. Now 19 factorial gets cancelled with 19 factorial and 2 into 10 is 20 and this is equal to 21 into 10 which is equal to 210. Hence the number of quads are 210. So this completes the question. Hope you enjoyed this session. Goodbye and take care.