How do you do this- You can use just one question and use it as an example. I am

 idk why they want the dratted "k" . it is so simple w/o it ! 
in the form y = ab^x, a = 4000, b = 4400/4000 = 1.1, x = t/2 
N = 4000*1.1^(13/2) = 7432 < 
 
now you won't be satisfied with simplicity, so 
4400 = 4000e^2k 
e^2k = 1.1 
k = ln 1.1/2 = .0477 to 4 dp 
N(13) = 4000e^(13*.0477) = 7436 < 

note that apart from a roundabout way, you get a less accurate ans !