Explanation:
did you change the reference number ?
I think the original question was "90 is how many % of 80 ?"
now it is 81.
in % calculations the most important things to identify :
what are 100% (or any other reference % level).
then what is 1%.
and then multiply this 1% by the desired number of % or divide the target number by this 1% to find out how many % the target number is.
in our case here
100% = 81
1% = 100%/100 = 81/100 = 0.81
how many % are 90 ?
well, as many as how often 1% fits into these 90 :
90/0.81 = 111.111111111... %
when the reference was 80, then we had
90/0.8 = 112.5%