Why are alkyl halides more soluble in alcohol water solution rather than pure water?
As others have mentioned, it generally comes down to "like dissolves like".  Water is highly polar, low molecular weight alcohols like ethanol a bit less so.  By increasing the ratio of alcohol to water in a solution you are effectively making the solution less polar as a whole.  There are effectively fewer hydrogen-bonding opportunities in an alcohol water solution than in pure water.  In general, alkyl halides tend to fall in the low-to-intermediate polarity range.  Methylene chloride for example is only very slightly soluble in water, which makes it an excellent candidate for performing extractions.  By adding alcohol to water, you are giving the solution as a whole a more non-polar character, and increasing the potential for favorable interactions between the alkyl halide and the solvent.