Teacher: In this task, you are given two sets, and you need to count the number of elements at the intersection of two given sets.  A Set is shown by two curly braces and comma-separated numbers inside, like {1, 2, 3}. The intersection of two given sets is the largest set which contains all the elements that are common to both sets. To find the intersection of two given sets, A and B is a set consisting of all the elements common to both A and B.
Teacher: Now, understand the problem? If you are still confused, see the following example:
Set1: '{11, 4, 6}', Set2: '{2, 3, 4, 10, 11, 20}'. How many elements are there in the intersection of Set1 and Set2 ?
Solution: 2
Reason: Intersection of Set1 and Set2 is {11, 4} and it has 2 elements. So, the answer is 2.

Now, solve this instance: Set1: '{2, 4, 6, 8, 9, 15, 16, 17, 19, 20}', Set2: '{7, 8, 9, 14, 18, 20}'. How many elements are there in the intersection of Set1 and Set2 ?
Student:
3