If the teacher were to take the number of students who only participated in math (40) and the number of students that participated in tutoring as a whole (40 + 55 + 47 + 58 = 200). She will get the percentage of people that were in math.
She can then multiply the 100 students by the percentage of students that participated in math to predict about how many of the pool of 100 will be in math tutoring.
40 / 200 = 4 /20 = 1 / 5 = 0.20 = 20%
100 x 0.20 or 20% of 100 = 20.
She can expect about 20 students out the whole 100-student pool to sign up and participate in math tutoring.