$\begin{align}  & \text{Let the set of those that offer Biology be }B \\ & \text{Let the set of those who offer Mathematics be }M \\ & n(B\cup M)=220 \\ & n(B)=125 \\ & n(M)=110 \\ & n(B\cap M)=x \\ & n(B\cap M{{}^{1}})=125-x\text{    (Those who Biology but not Maths)} \\ & n({{B}^{1}}\cap M)=110-x\text{    (Those who Maths but not Biology)} \\ & n(B\cup M)=n(B\cap M{{}^{1}})+n({{B}^{1}}\cap M)+n(B\cap M) \\ & 220=125-x+110-x+x \\ & x=15 \\ & \therefore \text{Those who offer Biology but not mathematics are } \\ & n(B\cap {{M}^{1}})=125-15=110 \\\end{align}$