A mixture containing 16% of a drug is combined to a 28% of a drug to obtain a 15mL of 24% of a mixture. How many 16% of a drug should be added?
\[
\boxed{16 \%} + \boxed{28 \%} = \boxed{24 \%}
\]
\[
\boxed{x} \quad \boxed{15-x} \quad \boxed{15}
\]
\[
0.16(x) + 0.28(15-x) = 0.24(15)
\]
\[
0.16x + 4.2-0.28x = 3.6
\]
\[
0.12x = 0.6
\]
\[
x=5 \leftarrow \text{answer}
\]