**Concept:** The question tests on the basics of Counting

**Solution**

Let the total number of houses be = x

Number of houses with 3 or more floors = 30% of x = 0.3x

Number of houses with less than 3 floors = 1 - 0.3x = 0.7x

Also given that 0.7x = 240 + 0.3x

Then 0.4x = 240

x = 240/0.4 = 600

Option C