abcde is five digit number. Show that abcde is divisible by 7 if and only if the number abcd-(2*e)
is divisible by 7 (* means Multiplication)

Denote concatenation **xyzw by bold letters ,-**and multiplication xyzw by regular font.

n1 **= abcde= **10***abcd +e**

n2 =10*** (abcd -2e)=**10***abcd -20e**

**n1-n2= 21*e is divisible by 7.**

**Reading the 3 lines to fro makes it if and only if.**