Find the missing bank digit:

1 3 ? 6 2 7 9 1 5

Calculate what you get from the numbers you know:

1*7+3*3+x*9+6*7+2*3+7*9+9*7+1*3+5*9=

7+9+x*9+2+6+3+3+3+5 = 8 (mod 10)

so in order to get 0 mod 10, I need x*9 to end in 2. 8*9=72, so the missing digit is 8