What is (3.2 × 10^5)(2 × 10^-3)?

Multiply the coefficients and add the exponents of ten: $(3.2)(2)=6.4$ and $10^5\cdot10^{-3}=10^{5+(-3)}=10^2$. The product is $6.4\times10^2$, choice D, which equals $640$; check: $320{,}000\times0.002=640$. Choice A adds the coefficients ($3.2+2=5.2$) instead of multiplying. Choice B treats $10^{-3}$ as $10^3$ and gets $10^8$. Choice C makes the exponent $-2$, as if subtracting $5$ from $3$ in the wrong direction. Negative exponents shrink the number, so the exponent after adding $5$ and $-3$ must still be positive.