Applications of Linear Equations : Word Problem

A professor started a project with a group of students, 60% of whom were boys. Due to some unavoidable reason, six girls couldn't turn up, so the professor had to make some changes in the group. He admitted six boys. In doing so, the percentage of boys in the project increased to 75%. Find the number of boys and girls involved in the project initially.

I don't know what formula or eqaution I should use.
I was thinking maybe x=boys and y=girls thus
x(.60)+ y(.40)=total students and
(.60x+6)+(.40y-6)= number of students
I'm not sure if I set this up correctly.