The number is of the form 3⬜6⬜.8 ⬜⬜ where the digits 3, 6 and 8 are already used. The four boxes have to be filled with four distinct digits chosen from {0, 1, 2, 4, 5, 7, 9}.

(i) Greatest possible number:

To get the greatest number, we fill the boxes with the four largest available digits in decreasing order from the leftmost box: 9, 7, 5, 4.

So the boxes are filled as: 3967.854

Greatest possible number = 3967.854

(ii) Smallest possible number:

To get the smallest number, we fill the boxes with the four smallest available digits in increasing order from the leftmost box: 0, 1, 2, 4.

So the boxes are filled as: 3061.824

Smallest possible number = 3061.824