I'll assume
![\mathrm B](https://tex.z-dn.net/?f=%5Cmathrm%20B)
is completely dominant over
![\mathrm b](https://tex.z-dn.net/?f=%5Cmathrm%20b)
. Recall the Hardy-Weinberg equations:
![\begin{cases}p^2+2pq+q^2=1\\p+q=1\end{cases}](https://tex.z-dn.net/?f=%5Cbegin%7Bcases%7Dp%5E2%2B2pq%2Bq%5E2%3D1%5C%5Cp%2Bq%3D1%5Cend%7Bcases%7D)
where
![p](https://tex.z-dn.net/?f=p)
represents the allele frequency for brown fur, or the number of copies of the allele
![\mathrm B](https://tex.z-dn.net/?f=%5Cmathrm%20B)
within the population; and
![q](https://tex.z-dn.net/?f=q)
represents the allele frequency for white fur, or the number of copies of
![\mathrm b](https://tex.z-dn.net/?f=%5Cmathrm%20b)
. In the first equation, the squared terms refer to the frequencies of the corresponding homozygous individuals, while
![2pq](https://tex.z-dn.net/?f=2pq)
is the frequency of the heterozygotes.
We're told that 12 individual moles have white fur, so we know for sure that there are 12 homozygous recessive individuals, which means
![q^2=\dfrac{12}{100}=\dfrac3{25}\implies q=\dfrac{\sqrt3}5\approx0.346](https://tex.z-dn.net/?f=q%5E2%3D%5Cdfrac%7B12%7D%7B100%7D%3D%5Cdfrac3%7B25%7D%5Cimplies%20q%3D%5Cdfrac%7B%5Csqrt3%7D5%5Capprox0.346)
from which it follows that
![p=1-\dfrac{\sqrt3}5\approx0.654](https://tex.z-dn.net/?f=p%3D1-%5Cdfrac%7B%5Csqrt3%7D5%5Capprox0.654)
Over time, H-W equilibrium guarantees that the allele frequencies
![(p,q)](https://tex.z-dn.net/?f=%28p%2Cq%29)
do not change within the population. For example, suppose we denote the frequency of the
![\mathrm B](https://tex.z-dn.net/?f=%5Cmathrm%20B)
allele in generation
![n](https://tex.z-dn.net/?f=n)
by
![p_n](https://tex.z-dn.net/?f=p_n)
. Then
![p_n={p_{n-1}}^2+\dfrac{2p_{n-1}q_{n-1}}2](https://tex.z-dn.net/?f=p_n%3D%7Bp_%7Bn-1%7D%7D%5E2%2B%5Cdfrac%7B2p_%7Bn-1%7Dq_%7Bn-1%7D%7D2)
That is, the frequency
![\mathrm B](https://tex.z-dn.net/?f=%5Cmathrm%20B)
in the
![n](https://tex.z-dn.net/?f=n)
-th generation has to match the frequency of
![\mathrm B](https://tex.z-dn.net/?f=%5Cmathrm%20B)
attributed to the
![\mathrm{BB}](https://tex.z-dn.net/?f=%5Cmathrm%7BBB%7D)
and half the
![\mathrm{Bb}](https://tex.z-dn.net/?f=%5Cmathrm%7BBb%7D)
individuals of the previous generation.
![p_n={p_{n-1}}^2+p_{n-1}q_{n-1}=p_{n-1}(p_{n-1}+q_{n-1})=p_{n-1}](https://tex.z-dn.net/?f=p_n%3D%7Bp_%7Bn-1%7D%7D%5E2%2Bp_%7Bn-1%7Dq_%7Bn-1%7D%3Dp_%7Bn-1%7D%28p_%7Bn-1%7D%2Bq_%7Bn-1%7D%29%3Dp_%7Bn-1%7D)
The same goes for
![q_n](https://tex.z-dn.net/?f=q_n)
.