Answer: 20 gallons of 16% and 8 gallons of 23% solution is usedfor 28 gallons of the 18 % solution.
Explanation:
According to the dilution law,
where,
= concentration of ist disinfectant solution = 16 %
= volume of pure acid solution = x gallons
= concentration of another disinfectant solution= 23%
= volume of another acid solution= (28-x) gallons
= concentration of resulting disinfectant solution = 18 %
= volume of resulting acid solution = 28 gallons

(28-x) = (28-20) = 8 gallons
Thus 20 gallons of 16% and 8 gallons of 23% solution is usedfor 28 gallons of the 18 % solution.