how do I divide -50db by 20?

If these are decibels and it is the original signal, S, that is 20 times smaller, then we would have either:

(1) -50 = 10log(S), so   x = 10log(S/20) = 10log(S) - 10log(20) = -50 - 13 = -63db  if the signal is an amplitude, or

(2) -50 = 20log(S), so x = 20log(S/20) = 20log(S) - 20log(20) = -50 - 26 = - 76db if the signal is a power

However, it isn't clear to me from the question if it is this interpretation or Melody's that is wanted!

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$$\frac{-50bd}{20}=\frac{-5bd}{2}$$

If you wanted to this could also be written as -2.5bd