$4.32 / 12 = $0.36 (12 pack)
$7.80 / 20 = $0.39 (20 pack)
The 12 pack of juice works out to cost $0.36 per box of juice while the 20 pack of juice works out to cost $0.39 per box of juice therefore, the 12 pack of juice is the best buy as you will pay less per box of juice than you would with the 20 pack of juice