the 100,000,000 eight-digit serial numbers from 00000000
to 99999999, there are 10,000
that read the same forwards and backwards.
Considering this fact, you might
expect 1 in every 10,000
bank notes to be a radar bill.
Since there are eight digits, the serial
numbers can range from 00000000 to 99999999. In other
words, there are 100,000,000 possible serial numbers.
Think of the eight digits as two groups of four. The
last four digits of a radar bill serial number determine
the entire number. For example, if the last four digits
are 0001, then the entire number must be 10000001.
Since the four digits that determine the radar bill serial
number range from 0000 to 9999, there are 10,000 possible
radar bill numbers.
Since there are 10,000 possible radar bill numbers out
of 100,000,000 serial numbers, you might expect 1 of every
10,000 bills to be a radar bill.
Another way to think about this is to look at the following
of Digits in Bill
of Radar Bills
Following this pattern, there could be 10,000 radar bills
that have eight digits.