2.3=(1.05)^n

2.3=(1.05)^n     take the log of each side

log 2.3 = log 1.05^n    and by a log property, we can write

log 2.3  = (n) log 1.05     divide both sides by log 1.05

log 2.3 / log 1.05  = n = about 17.07