We know that
• They sold to one customer 5 peanut butter biscuits for $3.
,
• To another customer, 7 beef treats for $4.20.
Using the given information, we form the following proportion
![(5)/(3)=(7)/(4.20)](https://img.qammunity.org/2023/formulas/mathematics/college/quzepnj7l5lz4lk6vmf6ogcsjgnfte0rg3.png)
Then, we divide to see if these ratios are equivalent
![1.66\ldots=1.66\ldots](https://img.qammunity.org/2023/formulas/mathematics/college/i1pqi21e0o6qz7hqle6eeje150cewocxc0.png)
As you can see they are equal.
The cross product of the proportion would be
![\begin{gathered} 5\cdot4.20=7\cdot3 \\ 21=21 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/11ws39zdc068xwefzt0ujqidwdgwxk27no.png)
Therefore, the cross product is equal to 21.