- Wat is de omtrek van het velletje papier waarmee ze begonnen?
A. 40 cm B. 50 cm C. 60 cm D. 80 cm E. 90 cm
\( \begin{array}{l} \left\{ \begin{array}{l} 2a + b = 40 \\ a + 2b = 50 \\ \end{array} \right. \\ \left\{ \begin{array}{l} 2a + b = 40 \\ 2a + 4b = 100 \\ \end{array} \right. \\ \left\{ \begin{array}{l} 3b = 60 \\ 2a + b = 40 \\ \end{array} \right. \\ \left\{ \begin{array}{l} b = 20 \\ a = 10 \\ \end{array} \right. \\ \end{array} \)
