Take a look at the shapes and you will see the same one repeating. If you work out the maths for each double digit number the pattern will be obvious...
eg/
10-1 = 9
11-2 = 9
12-3 = 9
etc
but all of my numbers were not nine. except one
1. 16 1+6=7-16=9 okay it got me there
2. 20 2+0=2-20=18 got me there too
3 88 8+8=16-88+72 there too
unless im being fooled, which is probably the case.