Let A be the set of 6 - digit numbers where the first three digits are all the same.
Similarly define B, C and D for the second, third and fourth sets of three consecutive digits.
|A|=|B|C|D|=10,000, since for each number with three consecutive equal digits, we have ten choices for the repeated digit and ten choices for each of the remaining three digits.
ANB=BNC=CND = 10,000, since each intersection represents sets of numbers with four consecutive equal digits.
|ANCI=BND=100, since these two intersections represent sets of numbers with five consecutive equal digits.