Memorize: log_10(2) = 0.3010, log_10(3) = 0.4771, log_10(7) = 0.8451 (less common). Derived: log_10(4) = 20.3010 = 0.6020, log_10(5) = log_10(10/2) = 1 - 0.3010 = 0.6990, log_10(6) = log_10(2) + log_10(3) = 0.7781, log_10(8) = 30.3010 = 0.9030, log_10(9) = 2*0.4771 = 0.9542. These are needed for digit-counting and approximation problems.