find the volumes og each box
Single serving box = 1/2 * 7 *3 * 9 = 94.5 cm^3
Family size = 1/2 * 8 * 3 * 16 = 192 cm^3
Find the cost by unit volume:- single costs 1.25 / 94.5 = 0.013228
family costs 2.5 / 192 0.01302
So the family bar costs less by unit volume and is the better buy.