so in this case
24:6.80
and 12:x
x= cost of 12 pack of water
set them straight up as "fractions" and cross multiply and then set each answer equal to each other
24 12
6:80 x
24x=81.6
divide both sides by 24
x= 3.40
it would cost $3.40 for a 12 pack of water
also, if since they are proportional, you could have just divided the price by 2 since the quantity was half of the original. But this is a safe method
