Gonna have to make an assumption as there's not enough info to answer the question - I'll assume that the whisy is 40% ABV
40% of 50ml = 20ml
i.e. the whisky is effectively made up of 20ml pure (100%) alcohol and 30ml neutral liquid.
Top it up to a pint (568ml).
The pint now contains 20ml of pure alcohol and 548ml of neutral solution.
Therefore the % ABV = 20/568 x 100 = 3.52%
...... I think lol :)