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.