Question 550577
Let quantity of 20% alcohol solution be added : x liters

Eqn for altered solution ....

> x*20% + 40 * 50% = (x+40) * 30%

> x*0.2 + 40*0.5 = (x+40)*0.3 ..(converting percentages into decimal)  
 
> 0.2x + 20 = 0.3x + 12
> 20-12 = 0.3x - 0.2x (Rearranging similar terms)
> 8 = 0.1 x
Hence x = 8/0.1 
Ans 80 Litres