When a small amount of impurity atoms are added to a semiconductor,then generally its resistivity

For an intrinsic semiconductor,where $n_{h}$ and $n_{e}$ are the number of holes per unit volume and the number of electrons per unit volume respectively,which of the following relations holds true?

The conductivity of a pure semiconductor can be increased by

$A$ pure $Si$ crystal has $5 \times 10^{28}$ atoms $m^{-3}$. It is doped with $1 \, ppm$ concentration of pentavalent $As$. The number of holes is (Take $n_i = 1.5 \times 10^{16} \, m^{-3}$)

If one indium atom is doped per $5 \times 10^7$ silicon atoms,and the number density of silicon atoms is $5 \times 10^{28} \, \text{atoms}/\text{m}^3$,find the number density of acceptor atoms in $\text{atoms}/\text{cm}^3$.

In a semiconductor,the ratio of the number of electrons to the number of holes is $7/5$ and the ratio of their currents is $7/4$. What is the ratio of their drift velocities?