(2.5×10⁻⁸)(9×10⁻¹⁰) = 2.5×9×10⁻⁸⁻¹⁰ = 22.5×10⁻¹⁸
= 2.25×10⁻¹⁷
_____
The rules of exponents apply.
... 10^a × 10^b = 10^(a+b)
... 22.5 = 2.25 × 10¹
