For this we just want to set it to a proportion so I hope this helps!
[tex] \frac{x}{800} = \frac{12}{100} [/tex]
From there, cross multiply, therefore 800 by 12 and 100 by x.
[tex]9600 = 100x [/tex]
[tex]Divide\;both\;sides\;by\;100\ \textgreater \ \frac{9600}{100} = \frac{100x}{100} \ \textgreater \ 96[/tex]
Therefore 96 students attended the Chess Club.
Hope this helps!