1. The problem states that $b \times 10^n$ is the number 7200 written in standard form. 2. First, write 7200 in standard form: $7200 = 7.2 \times 10^3$. 3. From this, we identify $b = 7.2$ and $n = 3$. 4. We need to find $b \times 10^{-n} = 7.2 \times 10^{-3}$. 5. Calculate $7.2 \times 10^{-3} = 7.2 \times 0.001 = 0.0072$. 6. Therefore, $b \times 10^{-n} = 0.0072$ as an ordinary number.