cooking

Let  s  be the number of grams of salt in the solution.

 

We want....

 

number of grams of salt / number of grams of solution   =   10 / 100

 

s / (810 + s)   =   10 / 100

                                               Multiply both sides of the equation by  810 + s .

s   =   (810 + s) * 10 / 100

                                               Distribute the  10/100 .

s   =   81 + 0.1s

                                               Subtract  0.1s  from both sides.

0.9s   =   81

                                               Divide both sides by  0.9 .

s   =   90

 

So Ilya needs to add  90  grams of salt.

 

That makes  90 + 810  =  900 grams of solution.