The circumference of a circle is 15π units. How many units long is its diameter? (Use only the digits 0 - 9 and the decimal point, if needed, to write the answer.)

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?