Mobile number: first 4 digits = 9,9,7,9. Make smallest number using one digit twice from 8,3,5,6,0.
Digits = 0,3,5,6,8 (with 0 used twice)
First 4 digits are fixed: (9,,9,,7,,9).
Available digits for the remaining 6 places: (8,,3,,5,,6,,0).
We are allowed to repeat exactly one digit once.
To make the smallest number, repeat the smallest digit.
Smallest digit here is (0).
So the 6 digits we will use are:
(0,;0,;3,;5,;6,;8).
Arrange these 6 digits in ascending order (since the first 4 are fixed):
(0,0,3,5,6,8).
Write the full number by placing them after the fixed prefix:
(9,9,7,9;0,0,3,5,6,8).