Well because there is a total of 55 students that will be your denominator
And there are 22 seventh graders so that would be you numerator since you want to know the percentage of the seventh graders present in the club.
To do so you just need to divide 22 by 55 and multiply that quotient by 100 and slap on that percent sign:
100 * (22 / 55) = 40%