i=0
j=0
sum_intensity=0
list_of_average_intensities = []
number_of_pictures = 3
intensities=[1,2,3,4,5,6,7,8,9]
for i in range(number_of_pictures):
    for j in range(i,len(intensities), number_of_pictures):
            sum_intensity = sum_intensity + intensities[j]
    average_intensity = sum_intensity/number_of_pictures
    list_of_average_intensities.append(average_intensity)
    sum_intensity = 0
print list_of_average_intensities