Question :

               2 x 3 = 18

               3 x 4 = 72

               Then

               4 x 5 = ?

Answer :

2 x 3 = 6 x 6/2 = 6 x 3 = 18

3 x 4 = 12 x 12/2 = 12 x 6 = 72

So the missing number is 200:

4 x 5 = 20 x 20/2 = 200