5. Let the cost of a pound of rice be x coins, a pound of brown sugar be y coins, and a pound of refined sugar be z coins. We obtain the system:

$$
\left\{\begin{array}{l}
4 x+\frac{9}{2} y+12 z=6 ; \\
12 x+6 y+6 z=8
\end{array}\right. \text { Subtract the first equation from twice the second equation and }
$$

express y in terms of x. We get $y=\frac{4}{3}-\frac{8}{3} x$. Substitute into the second equation and express $\mathrm{z}$ in terms of $\mathrm{x} z=\frac{2}{3} x$. Find the cost of the purchase:

$$
4 x+3 y+6 z=4 x+4-8 x+4 x=4
$$
Answer: 4 pounds sterling.