An equation of the form ax+by=c , where a, b, and c do not equal zero, is a direct variation.
a- sometimes
b- always
c- never

Becca wants to mix fruit juice and soda to make a punch. she can buy fruit juice for $3 a gallon and soda for $4 a gallon. if she wants to make 28 gallons of punch at a cost of $3.25 a gallon, how many gallons of fruit juice and how many gallons of soda should she buy?