Ans: A
Let $\$x$ and $\$y$ be the costs of the gained bag and the lost bag respectively.
Let $\$x$ and $\$y$ be the costs of the gained bag and the lost bag respectively.
For the gained bag,
$\begin{array}{rcl}
x\times(1+20\%) & = & 240 \\
x & = & 200
\end{array}$
Therefore, the profit is $\$40$.
For the lost bag,
$\begin{array}{rcl}
y\times(1-20\%) & = & 240 \\
y & = & 300
\end{array}$
Therefore, the loss is $\$60$. Hence after the two transactions, Mary lost $\$20$.
