Well, there are approximately 11.7 billion one dollar bills currently in circulation. Serial numbers, like any other number combinations, range from 0-9 (obviously). The number of "possible 8 digit combinations of 0-9" is mathematically the number of unique sets of 8 numbers that are selected from a group of 10 and the order doesn't matter. This is expressed 10C8 and since this is the same number as choosing 2 numbers to be excluded, it is just 10 times 9 divided by 2. That makes 45 choices. There is also the number of combinations where numbers can't be reused. We call these permutations. This is expressed 10P8 and is just 10 times 9: 90 choices. Finally, allowing numbers in the serial number to be repeated. The number of 8 digit numbers using the digits 0-9. Each digit has 10 choices and there are 8 of them, so the answer is 108 = 100 000 000 choices. So taking all the possible serial number combinations and using a probability equation to find out what the chances are of you finding a one dollar bill with a serial number ending in 7 in both equation form or a probability percentage, we can come to the conclusion that I just wasted your time reading this and I have no fúcking clue because I didnt pay attention in math class. Hope this helped