Question

I am stuck.. =\ Anyone?
Dean wants to make a mixture that is 50% lemon juice and 50% lime juice. How much 100% lemon juice should he add to a juice that is 30% lemon juice and 70% lime juice to make 7 gallons of the 50% lemon/50% lime juice mixture?

A) 0.5 gallon
b) 1 gallon
c) 2 gallons
d) 2.5 gallons