Answer:
7
Step-by-step explanation:
it is a 16 digit credit card number. Therefore, start from the first digit and double alternating digits. Add them and the non doubled digits. should be divisible by 10. For double digits, they are added to make single digit.
9+3+8+3+9+4+7+5+6+1+5+3+4+3+3+x = 73+x
final number should be divisible by 10.
x=7
