How do you know when to use log and natural logarithm?

The book answer is correct for each problem. For the 1st one, you made a simple error: 70 - 25 is not 50. For the 2nd, you doubled instead of halved the resultant mass.

12 = 24e^(-0.02t) 0.5= e^(-0.02t) ln(0.5) / -0.02 = t so t? 34.7?

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.