According to the Stefan-Boltzmann law, the rate of cooling is proportional to the fourth power of the temperature difference:
\[
\text{Rate of cooling} \propto (T^4 - T_{\text{ambient}}^4)
\]
Let the rate of cooling at 600 K be \( H \). Then, we can write:
\[
H \propto (600^4 - 300^4)
\]
Now, the rate of cooling at 900 K is:
\[
\text{Rate of cooling at 900 K} \propto (900^4 - 300^4)
\]
Using the ratios, we can solve for the rate of cooling at 900 K:
\[
\frac{900^4 - 300^4}{600^4 - 300^4} = \frac{16}{3}
\]
Thus, the rate of cooling at 900 K is \( \frac{16}{3} H \).