Question 341603
Let water required be x L
70%  antifreeze = 3.5 -x L
..
total 50% 3.5 L
..
0+0.7(3.5-x)=0.5*3.5
2.45-0.7x=1.75
-0.7x=1.75-2.45
-0.7x= -0.70
x= 1 L
1 liter 70% has to be drained and 1 L water added.