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Dear Yong,
If I understand the problem, what you are asking is what is x if
log(10) = log(30) - log(x) 
Whenever you see a "minus" sign in a logarithmic expression think "subtraction."  The given expression is mathematically identical to saying
10 = 30/x
x = 30/10 = 3
and that's the answer.  The beauty of this approach is that it is completely independent of the base of the logarithm.