18 mathematics, 5 which do take mathematics and biology...

if 8 take neither math nor biology then that number either has nothing to do with 18 taking mathematics or there is an error in the problem statement

based solely on those who actually take mathematics:

18 - 5 = 13

there 13 who take mathematics but do not take biology but the problem statement is not consistent unless there is some semantic difference between "mathematics" and "math"