Answer
$$6.25 \times 10^{-10} .$$
Work Step by Step
We simplify by raising each factor to the exponent and then we divide:
$$ \left(\frac{2.5 \times 10^{-9}}{5 \times 10^{-7}}\right)^4= \frac{(2.5 \times 10^{-9})^4}{(5 \times 10^{-7})^4}=\frac{39.06 \times 10^{-36}}{625 \times 10^{-28}}
\\
=\frac{39.06 }{625 }\times 10^{-36-(-28)}=0.0625 \times 10^{-8}\\ =6.25 \times 10^{-10} .$$
