your calculator probably only evaluates in base 10 (ln is base e but we won't use that today)
so [tex]log_a(b)=\frac{log_c(b)}{log_c(a)}[/tex] where c is the new base so if you wanted in base 10 [tex]log_4(20)=\frac{log_{10}(20)}{log_{10}(4)}[/tex] using calculator, that is about 2.16096
