Two spherical camphor particles of radii $20$ $cm$ and $5$ $cm$ far away from each other, are undergiong sublimation in a stream of air. The mass transfer coefficient is proportional to $1\sqrt{r\left ( t \right )}$, where $r(t)$ is the radius of the sphere at time $t$. Assume that the partial pressure of camphor far away from the surface of the particle is zero. Also, assume quasi-steady state, identical ambient conditions, and negligible heat effects. If $t_{1}$ and $t_{2}$, are the times required for complete sublimation of the $20$ $cm$ and $5$ $cm$ camphor particles, respectively, the ratio $t_{1}/t_{2}$ is _________________ (rounded off to one decimal place).