Rearrange as (1-1.5)*30000/15 = 1 - 1.5n
Then 1.5n = 1 - (1-1.5)*30000/15
Take logs of both sides and use log(ab) = b*log(a)
n*log(1.5) = log(1 - (1-1.5)*30000/15)
Divide both sides by log(1.5)
$${\mathtt{n}} = {\frac{{log}_{10}\left({\mathtt{1}}{\mathtt{\,-\,}}{\frac{\left({\mathtt{1}}{\mathtt{\,-\,}}{\mathtt{1.5}}\right){\mathtt{\,\times\,}}{\mathtt{30\,000}}}{{\mathtt{15}}}}\right)}{{log}_{10}\left({\mathtt{1.5}}\right)}} \Rightarrow {\mathtt{n}} = {\mathtt{17.039\: \!085\: \!832\: \!934\: \!851}}$$
n ≈ 17
.
Rearrange as (1-1.5)*30000/15 = 1 - 1.5n
Then 1.5n = 1 - (1-1.5)*30000/15
Take logs of both sides and use log(ab) = b*log(a)
n*log(1.5) = log(1 - (1-1.5)*30000/15)
Divide both sides by log(1.5)
$${\mathtt{n}} = {\frac{{log}_{10}\left({\mathtt{1}}{\mathtt{\,-\,}}{\frac{\left({\mathtt{1}}{\mathtt{\,-\,}}{\mathtt{1.5}}\right){\mathtt{\,\times\,}}{\mathtt{30\,000}}}{{\mathtt{15}}}}\right)}{{log}_{10}\left({\mathtt{1.5}}\right)}} \Rightarrow {\mathtt{n}} = {\mathtt{17.039\: \!085\: \!832\: \!934\: \!851}}$$
n ≈ 17
.