The unit cost (cost per pound) is given by the ratio between the amount paid and the amount purchased.
So, you have
[tex] \text{Cost per pound} = \dfrac{\text{Amount paid}}{\text{Amount purchased}} [/tex]
Solving this formula for the amount paid, you get
[tex] \text{Amount paid} = \text{Cost per pound} \cdot \text{Amount purchased} [/tex]
You can complete the first two rows using the first formula, and the last row using the last formula:
[tex]\begin{array}{c|c|c}15&11.80&0.79\\3&14.97&4.99\\2&11.98&5.99\end{array}[/tex]
