Let x be the amount (in liters) of the 10% solution that we need
4 (50%) + x (10%) = (4 + x)(20%)
Multiply both sides by 10 to cancel out the % sign
4(50) + x(10) = (4 + x)(20)
200 + 10x = 80 + 20x
Combine like terms
10x = 120
x = 12 Liters
Hope this helps :)
