well, Matti's house is a triangular prism, and to get the volume of it, we simply get the area of the triangle upfront and multiply by its length of 15.

Yes that is true, all opposite sides would be parallel, and all the corners are 90 degrees, then yes it is true
To the nearest thousand, population is 24,327,000
that means the population could range between 24,326,500 and 24,327,499