What is the check digit, d, for the credit card number shown below?
6424 9813 2720 008d
А. 1
B. 3
с. 5
D. 7

Mark each digit odd and even from the right side.
Now multiply each digit at an even place with '2'.And if the number after multiplication is double-digit then add both the digit to obtain a single digit.
Now add all the digits obtained in the previous step along with digits at odd places.
To this sum obtained check by adding what digit it becomes next multiple of '10'.And that digit will be our check digit.

Given:

The creditcard number: 6424 9813 2720 008d

To find :

Check digit,d

Solution:

6424 9813 2720 008d

Marking odd and even places from right side:

(e:even place,o:odd place)

e,o,e,o e,o,e,o e,o,e,o e,o,e,o

6 4 2 4 9 8 1 3 2 7 2 0 0 0 8 d

Multipling every even place digit with '2'.

12,4,18,2,4,4,0,16

If two digit number then add both digits to have single digit

12,4,18,2,4,4,0,16 → 1+2,4,1+8,2,4,4,0,1+6 → 3,4,9,2,4,4,0,7

Now adding all the digits obtained in prevoius step with digits at odd place:

(3+4+9+2+4+4+0+7)+(4+4+8+3+7+0+0)= 59

The next multiply of 10 can be obtained by adding 1 to '59' that is 60. So, the check digit will be'1'.

59+1 = 60

(60 is a multiple of the number '10'.)

The check digit is 1.

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