Logarithms 3: Problems using a single logarithm properties 

New set of Logarithm exercises to sit between the current Logarithms 2 and and the #18310 (Logarithms 4) exercises.

These exercises:

* Combine the skills from Logarithms 1 and Logarithms 2
* Test if the student can use logarithm properties with bases other than 10
* Test if the student knows the correct logarithm property to use in each scenario and when they can't be used (i.e when mixed bases)

http://sandcastle.khanacademy.org/pull/19052/