A protein molecule's diameter is measured as 42 Å using X-ray diffraction. If another measurement instrument gives the diameter in picometers (pm), and the ratio of the two measurements is $\frac{x}{10}$, what is the value of $x$?

The unit of expression ${{\rm{\mu }}_0}{{\rm{\varepsilon }}_0}$ are

The unit of expression ${{\rm{\mu }}_0}{{\rm{\varepsilon }}_0}$ are