SSC CGL Tier II 1 Dec 2016 Paper 1

(3) Let the 3–digit number be 100x + 10y + z.
Sum of the digits = x + y + z
According to the question,
Difference
= 100x + 10y + z – (x + y + z)
= 99x + 9y
= 9 (11x + y)
Clearly, it is a multiple of 3 and 9.