Since the number \\\\(21y5\\\\) is a multiple of \\\\(9\\\\).So, the sum of its digits \\\\(2 + 1 + y + 5 = 8 + y\\\\) is a multiple of \\\\(9\\\\).\\\\(\ herefore (8 + y)\\\\) is either \\\\(0\\\\) or \\\\(9\\\\) or \\\\(18\\\\) or ...Since \\\\(y\\\\) is a digit, so \\\\((8 + y)\\\\) must be equal to \\\\(9\\\\).i.e., \\\\(8 + y = 9\\\\)\\\\( \\Rightarrow y = 9 -8 = 1\\\\)