Name your desired number as x. If 20% of x is equal to 81, we can write:
\(0.20x = 81\)(do you see how I got here?)
The way you reach this expression is because 20% is equivalent to 20 * 1/100, which is just 20/100, which is equivalent to 0.20
Then, we realize that if we multiply this by 5, we just get:
That gives us our desired answer.