A60litres solution of alcohol and water contains 20liters of alcohol .how much alcohol must be added to produce a solution of 50% alcohol

Total quantity of solution containing alcohol and water = 60 liters
Amount of alcohol in the solution = 20 liters
Percentage of alcohol in the solution = (20/60) * 100
                                                           = 100/3
                                                           = 33.33 percent
So
33.33 percent alcohol is = 20 liters
Then
50 percent alcohol = [20/(100/3)] * 50
                               = [(20 * 3)/100] * 50
                               = (60 * 50)/100
                               = 3000/100
                               = 30 liters
So 30 liters of alcohol should be added to make the percentage of alcohol 50% in the solution.

D.