Answer
10 litres of 20% solution.
40 litres of 70% solution.
Work Step by Step
Let $x$ be the volume of 20% solution needed.
Let $y$ be the volume of 70% solution needed.
Because 50 litres is needed,
$x+y=50$
20%=0.2; 60%=0.6; 70%=0.7
To make 50 litres of 60% solution using 20% and 70% solutions,
$0.2x+0.7y=0.6(50)$
$0.2x+0.7y=30$
$x+y=50$
$0.2x+0.7y=30$
$x+y=50$
$x=50-y$
$0.2x+0.7y=30$
$0.2(50-y)+0.7y=30$
$10-0.2y+0.7y=30$
$10+0.5y=30$
$0.5y=20$
$y=40$
$x+y=50$
$x=50-y$
$x=50-(40)$
$x=10$
$x=10$
$y=40$
10 litres of 20% solution.
40 litres of 70% solution.