Let [tex]x[/tex] be the amount (by volume) of the first juice drink, and [tex]y[/tex] the amount of the second that you will use in the mixture.
We want a mixture of 20 L, so
[tex]x+y=20[/tex]
For each liter of the first drink, 0.25 L will be orange juice. For each liter of the second drink, 0.05 L will be orange juice. We want the new mixture to end up with 0.15 L of orange juice for every liter of the mixture, so that
[tex]0.25x+0.05y=0.15(x+y)=3[/tex]
Now we solve the system:
[tex]\begin{cases}x+y=20\\0.25x+0.05y=3\end{cases}\implies x=10,y=10[/tex]
