We will be solving a system of 2 equations and 2 variables in this problem. I am not sure with how comfortable you are with algebra and which part in particular you are stuck on, but I will set up and solve the equations so you can see. Feel free to follow up with questions about each step if you need help seeing how I got there:
Let x= the amount of 0% fat milk
Let y= the amount of 5% fat milk.
x+y=15 <--since we are making 15 pints of milk total
0*x+.05*y = .03 * 15 <--the total fat content in each needs to add