There are 85 students in a class, 40 of them like math,31 of them like science, 12 of them like both

There are 85 students in a class, 40 of them like math,31 of them like science, 12 of them like both, how many don't like either.

We have the following equation:
Total Students = Students who like math + students who like science - students who like both + students who don't like neither.

Plug in our knowns, we get:
85 = 40 + 31 - 12 + Students who don't like neither
85 = 59 + Students who don't like neither

Subtract 59 from each side, we get:
Students who don't like neither = 85 - 59
Students who don't like neither = 26