How many cubes with a length of 1/10 of a unit can be used to pack a rectangular prism that has the dimensions 4/10 of a unit by 9/10 of a unit by 2/10 of a unit?

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?