Question 220726
Let 

x = Number of students in both classes



To find the number of students in only one or the other (not both) class, we subtract out the number that are in both (which is 'x') from the total.



In other words, 



Number that are in one or other (not both) = (Number in pre-algebra + Number in math enrichment) - (Number in both) 


105 = (100+30) - x


105 = 130 - x


105-130 = - x


- 25 = - x


25 = x


x = 25


So there are 25 students in both classes.