Observation rule: (52-25) △ 9 = 3 (73-37) △ 9 = 4 (63-36) △ 9 = 4 what's the rule? Come on Hurry!

Observation rule: (52-25) △ 9 = 3 (73-37) △ 9 = 4 (63-36) △ 9 = 4 what's the rule? Come on Hurry!

After calling any two digit, ten digit and one digit, the difference between the new two digit and the original two digit can be divided by 9, and the quotient is the difference between ten digit and one digit
For example, the original number = 10A + B and the new number = 10B + a
Difference between two numbers = 9a-9b = 9 (a-b)
(63-36) △ 9 = 3, not = 4, the original question is wrong

1/18+1/54+1/108+1/180+1/270+1/378+1/504+1/648+1/810+1/990

Divide the denominator of each fraction into two numbers, and multiply them as follows:
1 / 3 × 6 + 1 / 6 × 9 + 1 / 9 × 12 +. 1 / 30 × 33 the difference between the two numbers of each denominator is 3
=（1/3-1/6+1/6-1/9+1/9-1/12+.+1/30-1/33）×1/3
=（1/3-1/33）×1/3
=10/99

1.35 divided by 0.108 is equal to?
To process!!!

1.35÷0.108
=1350÷108
=12.5