the digits of all multiples of 9 (up to 540) add up to nine
then from 549 they add up to 18 (with a few exceptions like 603)
then 27, then 36 etc. ect.
AND the result of the addition also add up to 9 then 18 then 27 etc ad infinitum.
No need to be embarrassed mate....if I didnt have the fingers I wouldnt be able to count up to nine.....if one gets knocked off I will never be able to set me valves gaps ever again