Explanation:

Starting from the back
106– 70 = 36, 70- 45 = 25, 45 – 29 = 16, 29 – 20 = 9, 20 – 16 = 4, 16 – 15 = 1. 
Every difference is perfect square starting from 36 and decreasing. 
Thus the wrong number is 18, it should be 16.