Ans: B
The total cost of the $10\text{ kg}$ mixture
The total cost of the $10\text{ kg}$ mixture
$\begin{array}{cl}
= & \$80 \times 4 + \$40 \times 6 \\
= & \$560
\end{array}$
Therefore, the mixture cost per unit weight
$\begin{array}{cl}
= & \$560 \div 10 \\
= & \$56 \text{/kg}
\end{array}$
