let A be Annie's score, B be Belinda's score and C be Cassies score.

so A+B+C = 504At the end of the game, the total of their scores adds up to 504

A = B+B*10/100Annie scored 10% more than Belinda

C = 10/100*(A+B)while Cassie scored 60% of the combined scores of the other two

I think you can probably continue from here.