2008-II-13

Ans: C
The cost of $x\text{ kg}$ of rice of brand $A$

$\begin{array}{cl}
= & x \times 8 \\
= & \$ 8x
\end{array}$

The cost of $y\text{ kg}$ of rice of brand $B$

$\begin{array}{cl}
= & y \times 4 \\
= & \$ 4y
\end{array}$

For the cost of the mixture $\$5\text{/kg}$,

$\begin{array}{rcl}
\dfrac{8x+4y}{x+y} & = & 5 \\
8x + 4y & = & 5(x+y) \\
8x+4y & = & 5x + 5y \\
3x & = & y \\
\dfrac{x}{y} & = & \dfrac{1}{3} \\
x:y & = & 1:3
\end{array}$