BECE Mathematics 2021 Paper

Intersection ( $\Omega$ ) are elements you can find in both sets.$n$ in $n(A \cap B)$ means the number of elements. So you count how many elements are in $A \cap B$ once you are done finding the elements.$\;A=\{1,3,5,7,9,11,13,15\}$$\;B=\{3,6,9,12,15\}$As you can see 3,9 and 15 are the only elements that can be seen in both the sets $A$ and $B$$\;\text{ Hence }\; A \cap B = \{3,9,15\}$Since you are asked how many elements ( n ), you count how many elements are in $\{3,9,15\}$ and I believe you've counted threeHence $n(A \cap B)=3$