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}$$\frac{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.