Begin with a similar, but
simpler problem. Suppose there were only two digits in
a serial number. In this case, a radar bill would have
to have a serial number with two identical digits: 00,
11, 22, 33, 44, and so on.
There are a total of 10 of these serial numbers out of
the 100 possible 2-digit numbers. If there were four digits,
the serial numbers would range from 0000 to 9999. Think
of the four digits as two groups of two. If the last two
digits were 01, the number of the radar bill would be
1001. How many radar bills would there be altogether for
four-digit serial numbers?