As 15 = 30 / 2 you can express it as 10 * 3 / 2

Log 10 = 1, and you have log 3 and log 2 so use the standard combination laws: log xy = log x + log y, log (x/y) = log x / log y.

Others solved the same way - the tricksy bit here is the fact that you have powers of 10 in the mix.