We begin with 100 ml each of mixtures A and B .
When combining them in the ratio 2:3 to create mixture $C$, the volumes from $A$ and $B$ taken are:
$$
\frac{2}{5} \times 100=40 \mathrm{ml} \quad(\text { from } A), \quad \frac{3}{5} \times 100=60 \mathrm{ml} \quad(\text { from } B) .
$$
For mixture A, which contains 40 g of sugar per 100 ml , the amount of sugar in 40 ml will be:
$$
\frac{40}{100} \times 40=16 \mathrm{~g} .
$$
For mixture B, which contains 30 g of sugar per 100 ml , the amount of sugar in 60 ml will be:
$$
\frac{30}{100} \times 60=18 \mathrm{~g} .
$$
Next, we add an equal volume of milk ( 100 ml ), so the total volume of the final mixture doubles:
$$
100 \mathrm{ml}(\text { from A and B })+100 \mathrm{ml}(\mathrm{milk})=200 \mathrm{ml} .
$$
Now, the total amount of sugar in the final mixture is:
$$
16 \mathrm{~g}(\text { from } \mathrm{A})+18 \mathrm{~g}(\text { from } \mathrm{B})=34 \mathrm{~g} .
$$
To find the percentage of sugar in the final mixture, we calculate:
$$
\frac{34}{200} \times 100=17 \%
$$
