no wait, I made it needlessly complicated, I should have said, from 3x=15 we get log(3x) = log(15), hence log(3) + log(x) = log(5) + log(3), hence log(x) = log(5), so x = 5. I.e. I hid the more difficult division problem that 15/3 = 5 inside the first derivation. my bad. I hope this uses only addition now. (#51).