Should be able to do a fairly straightforward process to solve this one. If you assume that people and buildings are evenly distributed though-out your area, then you can take the percentage flooded times the total number of buildings and people to figure the amount lost. Since people and buildings cluster together, it would likely improve your result to treat the areas differently. If you know enough about it, can you separate the area into (maybe) three area types (dense, moderate, sparse) and weight them accordingly (still summing to your original number)? If so, use the exact same method but weight accordingly. Hope this helps.