Answer:
Explanation:
When you make a such solution the alcohol content remains same. Let the added volume is x. 
Then we have:
- x*60/100 + 60*30/100 = (x + 60)*40/100
 - 0.6x + 18 = 0.4x + 24
 - 0.6x - 0.4x = 24 - 18
 - 0.2x = 6
 - x = 6/0.2
 - x = 30 
 
The added volume is 30 liters