Calculating percentage composition after drying

A partially dried clay mineral contains 8% water. The original sample contains nearly 12% water and 45% silica. The percentage of silica in the partially dried sample is nearly

1. 50%
2. 49%
3. 55%
4. 47%

I know the formulas for finding percentage composition of elements in a given compound. But how am I to use them here? Please help.

Is not a matter of percentage of elements in a compound. Take a initial mass of $100~\mathrm{g}$. Initially you have $12~\mathrm{g}$ of water, $45~\mathrm{g}$ of silica and a rest of $43~\mathrm{g}$. If the final sample has $8\%$ of water, you still have $45~\mathrm{g}$ of silica and $43~\mathrm{g}$ of the rest. So by a simple calculation:
$$x~\mathrm{g}\text{(water)}\cdot 100 / (43 + 45 + x) = 8$$
From here you obtain the whole mass and the final $\%$ of silica: $47\%$ (answer d).
The answer is 47%, obtained as $\dfrac{8}{8 + 88} \times 100\% =47$