48 : 2 = 24 (мм) − радиус окружности с центром B.
$\begin{array}{cc}\begin{array}{l}\underset{¯}{\phantom{0}}48\\ \phantom{0}\underset{¯}{4}\\ \phantom{0}\underset{¯}{\phantom{0}}8\\ \phantom{00}\underset{¯}{8}\\ \phantom{00}0\end{array}& \begin{array}{l}\overline{)2}\\ \overline{\overline{)24}}\end{array}\end{array}$