If a mixture of 6% disinfectant solution is made from 7% and 2% disinfectant solutions, how much of each solution should be used if 30 gallons of 6% solution are needed?
a) 15 gallons of 7% solution and 15 gallons of 2% solution
b) 18 gallons of 7% solution and 12 gallons of 2% solution
c) 20 gallons of 7% solution and 10 gallons of 2% solution
d) 25 gallons of 7% solution and 5 gallons of 2% solution