Practice the missing number UPC problems from last time, and do these:

Missing numbers in check routing numbers:

 

1. Bank routing numbers use a 7-3-9 rule (mod 10). Use the bank routing number rule to check which of these codes could be valid (has a correct check digit):

2 1 5 8 7 2 9 4 6
2 1 1 8 7 2 9 4 6  
2 1 1 8 7 2 0 1 6  

2. Find the missing digit in each of these bank routing numbers:

? 1 3 7 8 2 4 6 1
2 ? 6 9 2 4 4 1 7
1 3 ? 6 2 7 9 1 5
4 2 6 ? 6 6 9 1 1  
8 8 1 5 ? 7 1 0 2  
6 4 1 3 8 ? 8 1 3  
4 0 2 2 5 0 ? 6 1  
9 0 1 1 5 8 9 ? 0  
1 3 3 8 6 5 4 4 ?  

And the question I forgot last time:

3. What are check digits good for? Why do they bother to put check digits on UPC codes and things like that?

Answer to question 3