# How do we calculate the number of angstroms in a millimetre?

With difficulty! $1$ $m m$ $=$ ${10}^{-} 3$ $m$; $1$ $\mu m$ $=$ ${10}^{-} 6$ $m$; $1$ $\text{angstrom}$ $=$ ${10}^{-} 10$ $m$.
So $20$ $m m$ $=$ $20 \times {10}^{-} 3 \cancel{m}$ $\times$ ${10}^{6}$ $\cancel{\mu m} \cdot \cancel{{m}^{-} 1}$ $\times$ ${10}^{4} \text{ angstrom} \cancel{\cdot \mu {m}^{-} 1}$ $=$ ?? $\text{angstroms}$.