User Story 2

Having done that, you quickly realize that the ingenious machine is not
in fact infallible. Sometimes it goes wrong in its scanning. The next
step therefore is to validate that the numbers you read are in fact
valid account numbers. A valid account number has a valid checksum. This
can be calculated as follows:

account number:  3  4  5  8  8  2  8  6  5
position names:  d9 d8 d7 d6 d5 d4 d3 d2 d1

checksum calculation:
(d1+2*d2+3*d3 +..+9*d9) mod 11 = 0
So now you should also write some code that calculates the checksum for
a given number, and identifies if it is a valid account number.