Suppose that X is a normally distributed random variable with a mean of 12 and a standard deviation of 4. Determine the value k, so that P(k < X)

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?