9. The number of all the girls in the class equals to the number of students who solved the problem. Indeed, by the condition, the number of students who solved the problem equals to the sum of the number of girls who solved it and the number of boys, who solved it. But the number of boys who solved the problem is the same as the number of girls who did not solve the problem. So, the number of all the girls in the class equals to the number of students who solved the problem.