Let's use the variable 'x' to represent the number of 30c cards and the variable 'y' to represent the number of 35c cards.
If the total price of the box is $5.80, we can write the equation:
![0.30x+0.35y=5.80](https://img.qammunity.org/2023/formulas/mathematics/college/gsx3p1ksgsqisactktxzj71bt86q2i47np.png)
If the number of 30c cards is six more than half the number of 35c cards, we have that:
![x=6+0.5y](https://img.qammunity.org/2023/formulas/mathematics/college/p5zv9k0dneirh6csxnonvitjucc1y1k39v.png)
Using this value of x in the first equation, we have:
![\begin{gathered} 0.3(6+0.5y)+0.35y=5.8 \\ 1.8+0.15y+0.35y=5.8 \\ 0.5y=5.8-1.8 \\ 0.5y=4 \\ y=(4)/(0.5)=8 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/xzju7pyr0w6kzaavfn2ovpj5xuquh8ldgs.png)
So the number of 35 cards is 8.