Don't be fooled by the first two groups 88-63=25 and 25-9= 16.

That's just fooling you.

The actual rule is: add the above two numbers separately.

It's the following number. For example:

88 - 63

|

25

The actual law is

8+8+6+3=25

Conversely:

2+5+9= 16

1+6+2+4= 13

1+3+4+9= 17

(A= 17)

1+7+ 1+0=9