+1 for a good question!
When I wanted to pick "C" here I said "wait a minute". Here, we have DS-pattern that is very similar to C-trap: the fist condition is obviously insufficient and help the second condition, and the second seems to be insufficient along.
So, I return to the question that says: How many of houses in the community have a swimming pool? (regardless having a patio).
If we read close the second condition we can reveal that the change of the number of houses in the community that have a patio and a swimming pool actually does not influence on the number of houses with a swimming pool.
Now, the same idea by formulas:
N=75 - the total number of houses
Np=48 - the number of houses with a patio.
Nps -the number of houses with both a patio and a swimming pool.
N(p)(s) -the number of houses without a patio or a swimming pool.
Ns - the number of houses with a swimming pool.
Ns=Nps + (75-48-N(p)(s)) = Nps + 27 - Nps = 27
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