The radius of a hydrogen atom is about $5.3\times 10^{-11}$ m. Which of these is the same length written with a different power of ten?

Moving the decimal point one place right multiplies the front number by 10, so the power of ten must drop by 1 to keep the value: $5.3\times 10^{-11}=53\times 10^{-12}$. Changing the power in the same direction as the digits gives $53\times 10^{-10}$, which is 100 times too big.