Comment by alipang

You don't have to memorize that log(5) = 0.7 if you know log(2) = 0.3 since log(5) = log(10 / 2) = log(10) - log(2) = 1 - 0.3 = 0.7.

Also log(8) = 0.9 because log(2^3) = 3log(2) = 0.3 3 = 0.9