Respuesta :
4 buildings and 4 colors.
First building can be 1 of 4 colors
Second building can be 1 of 3 colors because it can't be the same color as the first building.
Third building can be 1 of 3 colors because it can't be the same color as the 2nd building.
The forth building can be 1 of 2 colors because it can't be the same color as the first or third buildings.
Total combinations: 4 x 3 x 3 x 2 = 72 different ways.