x is the unknown number.
11 is 81 % of what number?
11 = 0.81 x
The equation to solve then is: 0.81 x=11
Solving for x, you must divide 11/.081
x=11 /0.81
we get 13.580247
x is 13.580247
This means, 11 is 81 % of 13.580247
To check yourself
13.580247 * 0.81=11