Basically, you're solving a proportion.
the proportion would be:
40/100 = 10/x
40 goes over 100 since it's a percent, and 10 and 40% are equal since 10 of the students in the art class represent 40% of the total number of students. X represents the missing info, which is the total number of students.
Now, you just cross multiply:
40(x) = 40x
10(100) = 1000
Since that's done, you now divide 1000 by 40, which equals 25.
There are 25 students in the art class :)
