ANSWER:

If you observe all the numbers you will notice that the numbers in the outer circles are the result of the multiplication of the inner numbers.

So,

3 x 15 = 45
6 x 30 = 180
7 x 35 = 245
4 x 20 = 80

So the number in the middle has to be a number which when multiplied by the adjacent number given the product in the outer circle.

So the number in the center has to be 5.

3 x 5 = 15
6 x 5 = 30
7 x 5 = 35
4 x 5 = 20

I hope you had fun finding the answer to the riddle!! Don’t forget to share it with your friends!!