This might be a little easier if we remember that the dB expression refers
to a multiplication.
10 dB loss means you multiply by 0.1.
Thinking of it this way, and remembering that multiplication is accomplished
by adding exponents (10^2 X 10^2=10^4), it's pretty easy to visualize that
if we multiply by 0.1 on the first 100 ft, then we do it again on the
second, 0.1 X 0.1=0.01, or down 20dB.
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