OK, I think this is it:
It all comes down to the subtraction of the two numbers -- your first number and then its transposition.
You then circle and remove one digit, which can’t be a zero..
Sum the digits remaining (after the one has been removed, which can’t be a zero)
Subtract that sum from 27
If that result is still greater than one digit, subtract 9 from that result.
Continue subtracting 9 from that result until you have one digit.
That one digit is the number you circled.
