First, lets start with the greatest 4-digit number. That would be 9,999.99999999999, with 9 to ∞, because the next number is a 5-digit number. The smallest 6-digit number is 100,000, because the number preceding 100,000 is a 4 digit number. To find difference, you have to subtract. 100,000 minus 9,999.9999 continued gives you 90,000.000000 to ∞ with a one with 1 at the end. You would write 90,000.01 with bar notation over only the 0 to indicate that it goes on to infinity.