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If a number is a multiple of 9, then the sum of its digit will be divisible by 9 .
Sum of digits of 31z5 = 3 + 1 + z + 5 = 9 + z
hence 9 + z have to be multiple of 9
this possible when 9 + z = 0, 9 , 18, 27 ...
since z is a single digit number this sum can only be 9 or 18 . therefore, z should be 0 or 9 .
hence two possible values of z = 0 or 9.