Given data:
Suppose Emma purchase $x amount of goods.
The final amount paid by Emma if she goes to left cashier is,
[tex]F=0.80x-10[/tex]The final amount paid by Emma if she goes to right cashier is,
[tex]\begin{gathered} F^{\prime}=0.80(x-10) \\ =0.8x-8 \end{gathered}[/tex]As the F value is less than F'
Thus, Emma must go to the left cashier in order for better deal.