Two blocks of masses m and 3m on a horizontal surface are in contact with the ends of a horizontal massless spring. The coefficient of friction between m and surface and between 3m and surface is $\mu$ and $\frac{\mu }{3}$ respectively. The two blocks are moved towards each other to compress the spring and then released. The two blocks move off in opposite directions covering distances $S_1$ and $S_2$ before coming to rest. $S_1:S_2$ is