$ \hat{A}+\hat{B}=90 , \hat{A}=3\hat{B}+10 \Rightarrow (3\hat{B}+10)+\hat{B}=90 \\ \Rightarrow 4\hat{B}+10=90 \Rightarrow 4\hat{B}=80 \Rightarrow\hat{B}=20 $