One sheet of a notebook is $1.5\times 10^{-4}$ m thick. What is the total thickness of 200 such sheets, in standard form?

Write 200 as $2\times 10^2$, then multiply: $1.5\times 2=3$ and $10^{-4}\times 10^2=10^{-4+2}=10^{-2}$, so the stack is $3\times 10^{-2}$ m, that is 0.03 m. Subtracting the exponents gives $3\times 10^{-6}$ m, which would make 200 sheets thinner than one.