For this case we propose a rule of three:
90 ----------> 200%
x ------------> 80%
Where the variable "x" represents the unknown number.
[tex]x = \frac {80 * 90} {200}\\x = \frac {7200} {200}\\x = 36[/tex]
So, we have the unknown number is 36.
Answer:
The unknown number is 36.
