A geyser heats water flowing at a rate of 2.0 kg per minute from $30^{\circ} \mathrm{C}$ to $70^{\circ} \mathrm{C}$. If geyser operates on a gas burner, the rate of combustion of fuel will be $\_\_\_\_$ $\mathrm{g} \mathrm{min}^{-1}$ [Heat of combustion $=8 \times 10^3 \mathrm{Jg}^{-1}$ Specific heat of water $=4.2 \mathrm{Jg}^{-1}{ }^{\circ} \mathrm{C}^{-1}$ ]