In a pure semiconductor, $n = p = n_i$, the intrinsic conductivity is: $\sigma_i = (n\mu_n+p\mu_p) e = (\mu_n+\mu_p) \times e\;n_i$. However, in the case of a doped semiconductor, the concentration of mobile charges is dependent solely on the concentration of doping atoms. In this case, $\sigma_p \approx N_a \mu_p e$

Since there are $5 \times 10^{28} \text{silicon atoms}\; m^{-3}$, the necessary concentration of acceptor atoms $N_a = 5 \times 10^{28} \times 10^{-7} = 5 \times 10^{21}$

$\Rightarrow \sigma_p = 5 \times 10^{21} \times 0.048 \times 1.6 \times 10^{-19} = 38.4 \; S/m$

$\Rightarrow$ Resistivity $ \rho_p= \large \frac {1}{\sigma_p} $

$\quad \quad = \large \frac {1}{38.4}$$ = 2300\; \Omega \;m$