Answer :
The unknown digit is 4 because you must have a number (in the space) that is less than five and more than four because looking at a number like
10 9 8 7 6 5 4 3 2 1 the only while number in between 5 and 3 is 4.
10 9 8 7 6 5 4 3 2 1 the only while number in between 5 and 3 is 4.
