A tablet weighs $2.5\times 10^{-4}$ kg and its sugar coating weighs $3.5\times 10^{-4}$ kg. What is the total mass in standard form?

The powers already match, so add only the decimal parts: $\left(2.5+3.5\right)\times 10^{-4}=6\times 10^{-4}$ kg. Adding the exponents gives $10^{-8}$ and multiplying them gives $10^{-16}$; $0.6\times 10^{-3}$ is the right value but is not standard form, since the digit before the point must not be 0.