Let x be the number of liters needed from the first acide concentration (20%)
Let y be the number of liters needed from the first acide concentration (60%)
We know at the end we will have:
x+y = 80 ; we also know that:
20%(x) + 60%(y) = 30%(80) OR 0.2x + 0.6y = 0.3(80) = 2x+6y=240
Hence we have :
x+y = 80
2x+6y = 240
Solving this system of 2 equations give: x = 60 and y = 20
In short you need to mix 60 litres at 20% and 20 liters at 60%