Let's use a Venn diagram to solve this problem. We have two sets: the set of students who offer Biology and the set of students who offer Mathematics. The total number of students in both sets is 220. The number of students who offer Biology is 125, and the number of students who offer Mathematics is 110. We can use the formula for the union of two sets to find the number of students who offer both Biology and Mathematics: |A ∪ B| = |A| + |B| - |A ∩ B|. Plugging in the values, we get:

220 = 125 + 110 - |A ∩ B|

|A ∩ B| = 15

So, there are 15 students who offer both Biology and Mathematics. Therefore, the number of students who offer Biology but not Mathematics is 125 - 15 = 110.