The kaplan book explains it well. What you do is first write the number in scientific notation like 2.0 x 10^-5

There is a log rule where log(2.0 x 10^-5) can be rewritten as -5+log(2.0) since log(10^x) = x. Since for our equation we are doing the -log our problem is 5-log n. So now we know 5 - log

. So the answer has to be around 5. But we also know that log can be either 0 if we do log(1) and 1 if we do log(10). So the log

term will be a number between 0 and 1. The bigger the number the closer to 1 it is and the smaller the number the closer to 0 it is.

So in the example above, 5-log(2.0). 2.0 is close to 0. 2/10 is 0.2. So we can estimate 5-0.2=4.8. If you did the answer on your calculator the answer is 4.7 so we are pretty close.