Two masses $M_{1} and M_{2} are tied together at the two ends of a light inextensible string that passes over a frictionless pulley. When the mass M_{2} is twice that of M_{1}, the acceleration of the system is a_{1}. When the mass M_{2} is thrice that of M_{1}, the acceleration of the system is a_{2}. The ratio \frac{a_{1}}{a_{2}}$ will be :