What is (8 × 10^7) ÷ (2 × 10^-2)?

Divide the coefficients and subtract the exponents of ten: $\frac{8}{2}=4$ and $10^{7}\div10^{-2}=10^{7-(-2)}=10^{9}$. The quotient is $4\times10^9$, choice D. Choice A ($4\times10^5$) subtracts the exponents as $7-2$, ignoring the negative sign. Choice C ($4\times10^{-9}$) has the exponent's sign wrong. Choice B ($1.6\times10^9$) multiplies the coefficients ($8\cdot2=16$) instead of dividing them. Division by a very small number makes the quotient larger, so the exponent should increase, to $9$. A quick estimate, $\frac{80{,}000{,}000}{0.02}$, is four billion.