Let the [tex]x[/tex] represent amount of salt in grams that Illya should add. Then, the following equation should be solved:
[tex]\frac{810}{90}=\frac{x}{10}[/tex]
[tex] \frac{x}{10}=9 [/tex]
[tex]x=9*10=90[/tex]
So, Illya should add 90g of salt to make 10% solution. The total solution that he make is 810g + 90g = 900g.
