Answer: 315 Adults.
Step-by-step explanation:
Let the number of adults is x.
Then the number of kids is (810-x).
The ratio of kids to adults is
810 - x/x = 11/7
It is your basic equation, which you get from the condition.
To solve it, cross multiply; then simplify
7*(810-x) = 11x
7*810 - 7x = 11x
7*810 = 11x + 7x
7*810 = 18x
x = 7 x 810/18 = 315.