1 we express a number as a linear combination of powers of 10
2 for example
1980=1*10^3+9*10^2+8*10^1+0*10^0
we get [1 9 8 1]
apply divisibility rule by 7
remaining digits-2(last digit)
k=1
1 9 8 -2(1)=196
k=2
19 - 2(6)=7
k=3
0-2(7)=-14 which is divisible by 7
stop we get a negative number