And also, what if there is another value s.t. X=9115/N is equal to .TBCDBCD..... such that T != 9?
You haven't shown that either, you're assuming there is only one answer.
[deleted an attitude comment]
I do not see any reason not to understand that n is/must be 9990 given that x = m/n = 9115/n.
Again with the given facts (i.e. the structire of 0.TBCDBCDBCD in which T is not repeating but BCD is), the denomenator, n, must be 9990.
In x = (TBCD-T)/9990 = m/n, m (or TBCD-T) cannot be greater than 9115 but could be smaller than that. The maximum m is a 4 digit number (zzzz or 9115) with 9 in thousand's place. Similarly (TBCD-T) is the max 4 digit number in neumarator for x = m/n. m could be 1823. In that case, when x = 1823/p where p = n/q, yes I agree that (TBCD-T) could not be 9115 but when m is clearly given 9115, (TBCD -T) is that 9115.
Therefore (TBCD-T) = 9115.
If it is still unclear, PM me for more detail repeating/terminating decimal.
It seems a real difficult question.