Two mass \( M_1 \) and \( M_2 \) are tied together at the two ends of a light inextensible string that passes over a frictionless pulley. When the mass \( M_2 \) is twice that of \( M_1 \), the acceleration of the system is \( a_1 \). When the mass \( M_2 \) is thrice that of \( M_1 \), the acceleration of the system is \( a_2 \). Then ratio \( \dfrac{a_1}{a_2} \) will be:
