## Elementary Technical Mathematics

$$6.25 \times 10^{-18} .$$
We simplify by raising each factor to the exponent and then by division we have $$\frac{(5 \times 10^{-6})^2}{4 \times 10^{6}}=\frac{25 \times 10^{-12}}{4 \times 10^{6}}=6.25 \times 10^{-18} .$$