Given that,
Shelley's pet food store sold one customer 5 peanut butter biscuits for $3. She sold another customer 7 beef treats for $4.20.
For peanut, the cost is ratio is
![(5)/(3)](https://img.qammunity.org/2021/formulas/mathematics/high-school/5iq5nbug1yq1yha80v4a7kggvjkdjh0hek.png)
For beef, the ratio is
![(7)/(4.2)](https://img.qammunity.org/2021/formulas/mathematics/high-school/xar65vpcof92sry9g6gb6f26kmpjmv4w2i.png)
If we take cross product of these items,
![(5)/(3)=(7)/(4.2)\\\\5* 4.2=3* 7\\\\21=21](https://img.qammunity.org/2021/formulas/mathematics/high-school/5yh0jwlz7a3pbjki44mui60y6v6t8fnnpc.png)
It implies, that the sale is in true proportion. From the cross product, we find that it is equal.