A $2 \mathrm{~m} \times 2 \mathrm{~m}$ tank of $3 \mathrm{~m}$ height has inflow, outflow and stirring mechanisms. Initially, the tank was half-filled with fresh water. At $t=0$, an inflow of a salt solution of concentration $5 \mathrm{~g} / \mathrm{m}^{3}$ at the rate of $2$ litre/s and an outflow of the well stirred mixture at the rate of $1$ litre/s are initiated. This process can be modelled using the following differential equation:
\[
\frac{d m}{d t}+\frac{m}{6000+t}=0.01
\]
where $m$ is the mass (grams) of the salt at time $t$ (seconds). The mass of the salt (in grams) in the tank at $75 \%$ of its capacity is $\_\_\_\_\_\_\_$ (rounded off to $2$ decimal places).