Dumb question about decibels

1. The problem statement, all variables and given/known data

Let E1 be the electric field 1cm into a person's skin and E0 be the electric field on the surface of that person's skin. If the field loses -486db/m and E0 = 64.1, find the value of E1.





2. Relevant equations



E1/E0 = k, with k < 1





3. The attempt at a solution



Well I'm mostly confused about the use of 20log vs 10log. My solution is:



We have -486dB/m, or -4.86dB/cm.



10log(E1) = 10log(k) + 10log(E0), where 10log(k) must be negative since k < 1.



We know that 10log(k) = -4.68, thus k = 10^(-4.68/10) = 0.3404



However, my teacher wrote the following equation:

20log(E1) = 20log(k) + 20log(E0)



Which gives a different value of k, obviously. I thought decibels were always 10log(ratio), and the only reason why 20log(ratio) sometimes appear is when you have a ratio of squared values, such as 10log(P1/P2) = 10log([V1^2/R]/[V2^2/r]) = 10log(V1^2/V2^2) = 20log(V1/V2) in electronics.



I'd like to know if 10log or 20log should be used in this situation. Thanks!