Let x be the number of houses under the question.


The rate of work of each builder is    in the first case, in terms of house per day for every builder.


The rate of work in the second case is  , in terms of house per day for every builder.


The rate is assumed to be the same in both cases, which gives you a proportion


     = .


From the proportion,


    x =  = 3.


ANSWER.  3 houses.

    Actually, you can get the answer without calculations (mentally).


    In the second case the number of workers is half of the first case, and the time is also half of that.


    So, the answer is  = 3 houses, same, as in the solution above.