You need to know the basics of log operations.  Here is a quick refresher

Remember the basics:
If e^POWER = NUMBER, 
then POWER = LN(NUMBER) 
and EXP(POWER) = NUMBER

LN and EXP are both Excel functions.  LN is the same as LOG, except that it is to the base e.  (ie, LN(x) = LOG_e(x) ).

Basic operations:
Adding logarithms is like multiplying numbers, and subtracting logs is like dividing numbers.

log_a(mn)=log_a(m) + log_a(n)
log_a(m/n)=log_a(m) - log_a(n)

Also,log_a(x^y )= y log_a(x)

Converting log bases:

If we know the log of a number in a particular base, we can convert it to another log base quite easily.

If x = log_a(m)
Then x =  log_b(m)/log_b(a)

What are logs useful for in finance?