A hospital patient is required to have at least 84 units of drug A and 120 units of drug B each day (assume that an overdose of either drug is harmless). Each gram of substance M contains 10 units of drug A and 8 units of drug B, and each gram of substance N contains 2 units of drug A and 4 units of drug B. Now, suppose that both M and N contain an undesirable drug D: 3 units per gram in M and 1 unit per gram in N. How many grams of each of substances M and N should be mixed to meet the minimum daily requirements and simultaneously minimize the intake of drug D? How many units of the undesirable drug D will be in this mixture?