Coz look the middle number in the previous two.....it is greater than first and third number..
And the third number is coming comfortable between the range of first and second number...so in this case third number would be between 13 and 17... So possible options are 14,15,16
It's correct. Look at the circle in halves. The left half gives you the upper values 33 and 55 by squaring the numbers and subtracting it. Similarly the right half gives you the value by squaring the greater number of the two and subtracting it from the other.