Two identically sized primary settling tanks receive water for $\text{Type-I}$ settling (discrete particles in dilute suspension) under laminar flow conditions. The Surface Overflow Rate (SOR) maintained in the two tanks are $30\ m^3/m^2.d$ and $15\: m^3/m^2.d$. The lowest diameters of the particles, which shall be settled out completely under SORs of $30\: m^3/m^2.d$ and $15\: m^3/m^2.d$ are designated as $d_{30}$ and $d_{15}$ respectively. The ratio, $\large \dfrac{d_{30}}{d_{15}}$ (round off to two decimal places), is ________.