To find the log of a number, there's a trick that kaplan teaches.

For example, if you want to find log 3.5 * 10^6, calculate it like this:

6 - log 3.5... you can estimate this to be around 5.7

Basically you take the exponent and make that the first number, then you take the first number and you have to estimate the log of it and find the difference. The answer choices for this type of problem will usually be really different, so it will be easy to tell which answer is the right one.

Hope this helps!!