Mod numbers and checks:

Some mod number questions:

Do the following calculations mod 7:

4+6=
3-5=
5x2=
6x4=
2-6=

Do the following calculations mod 11 (use X as a digit to represent the number 10):

8+2=
7+X=
2-9=
6-8=
4x5=
7x8=
9x6=
Xx5=

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

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 ?