Your math is wrong. Assuming that desired tax revenue should scale linearly with the number of families, the owners of the non-apartment parcels would be paying 1.35 times what they paid before the apartment building was built, not 1.5.
However, even ignoring all of the questionable assumptions that go into such an unrealistic scenario, the tax justifies itself since that neighborhood was clearly massively underdeveloped compared to its land value (i.e. housing demand).
However, even ignoring all of the questionable assumptions that go into such an unrealistic scenario, the tax justifies itself since that neighborhood was clearly massively underdeveloped compared to its land value (i.e. housing demand).