Calculate the direct voltage common mode rejection ratio for an instrument in which a common mode voltage of 100V produces an error in reading of 5mV
The amount of output voltage error (in your case 5mV) in proportion to the input common mode voltage (in your case 100V) is the common mode rejection ratio. This has nothing to do with time, so no need to specify some dV/dt.
\$ CMRR = \frac{V_{out}}{V_{com_i}} = \frac{5~mV}{100~V} = 50 \cdot 10^{-6}\$
As you can the, the CMRR (Common mode rejection ratio) has no unit, because it is a ratio.
Typically the value of the CMRR is not given as a linear number, but in the logarithmic measure of dB:
\$ CMRR_{dB} = 20 \cdot log(CMRR) = -86~dB \$
Notice that dB is not a unit! It only is scaling the linear value of the CMRR to a logarithmic scale.