In an art class 40% of the students used watercolor for paintings. ten students in the art class used watercolors. how many students are in the art class

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 :)