Answer :
The total amount Paul spent that month is the value of the discount card plus the value of the passes he bought.
He bought 24 passes each costing $1.6, therefore:
[tex]\text{passes}=24\cdot1.6=\text{ \$}38.4[/tex]The value of the passes added to the value of the card must be equal to the total spent.
[tex]\begin{gathered} \text{total}=\text{card}+\text{passes} \\ 42.8=\text{card}+38.4 \\ \text{card}=42.8-38.4 \\ \text{card}=4.4 \end{gathered}[/tex]Paul spent $ 4.4 on the student discount card.