500 = 400(1.05)^x divide both sides by 400
5/4 = (1.05)^x
1.25 = (1.05)^x take the log of both sides
log(1.25) = log(1.05)^x and we can write
log(1.25) = x * log(1.05) divide both sides by log(1.05)
log(1.25) / log(1.05) = x = about 4.5735
