Find the missing bank digit:

? 1 3 7 8 2 4 6 1

Calculate what you get from the numbers you know:

x*7+1*3+3*9+7*7+8*3+2*9+4*7+6*3+1*9=

x*7+3+7+9+4+8+8+8+9 = x*7+6 (mod 10)

so in order to get 0 mod 10, I need x*7 to end in 4. 2*7=14, so the missing digit is 2