Answer :
[tex]x-\ price\ of\ juice\\\\
12*x=48\ \ \ |Divide\ by\ 12\\
x=4\$\\\\
If\ she\ will\ buy\ 8\ more:\\
12+8=20\ packs\\\
20*4=80\$\\\\She\ will \ pay\ 80\$.[/tex]
[tex]12+8=20 \ juice \ boxes \ needs \ to buy \\\\ 12b.......48\$ \\20b.........x \\\\ x=\frac{20*48}{12} \\\\ x=20*4 \\\\ \boxed{x=80\$}[/tex]