The concentration of milk in mixture A is 40%, and the concentration of milk in mixture B is 75%. In what ratio should they be mixed so that 5L of the resulting mixture C has 3L of milk?

It is given that the concentration of mixture A is 40% and the concentration of mixture B is 75%. The resulting mixture C has 3L of milk in 5L of the mixture. So, the concentration of the mixture is calculated as,

Concentration of milk in C = $$\dfrac{3}{5}\ \times\ 100\ =\ 60\%$$

Using the allegation method, we can calculate the ratio in which they are mixed to be as,

So, the ratio at which they must be mixed is 15:20 = 3:4.

The correct answer is option B.