9. Milk and cream contain different percents of butterfat. How much $3\mathrm{\%}$ milk needs to be mixed with how much $15\mathrm{\%}$ cream to give 20 L of $6\mathrm{\%}$ cream?

Step 1 ：Let's denote the volume of the 3% milk as x (in liters) and the volume of the 15% cream as y (in liters).

Step 2 ：We can set up two equations based on the volume and the percentage of butterfat.

Step 3 ：The first equation is based on the total volume: $x+y=20$.

Step 4 ：The second equation is based on the total percentage of butterfat: $0.03x+0.15y=0.06\ast 20$.

Step 5 ：Solving these two equations gives us the values of x and y.

Step 6 ：The solution to the equations is $x=15$ and $y=5$.

Step 7 ：Final Answer: The solution to the problem is $\boxed{{\displaystyle x=15}}$ and $\boxed{{\displaystyle y=5}}$. This means that 15 L of 3% milk needs to be mixed with 5 L of 15% cream to give 20 L of 6% cream.