For question 15, so far, I have got the following

dN/N = -k dt

From there, I get N(0)e^{-kt}

I solved k to be 0.0001216 that was using k = -ln(1/2)/5700 ==> (ln 2)/5700

then N(t) = N(0)e^{-(ln}^{2)t/5700}

which is also N(t) = N(0)*-2^{t/5700}

So I am getting close, but what is N(0)?

so to do the part b I solve N(t) = N(0)*-2^{0.4(5700)}- but what is N(0)?

Question 16 I have solved, don't have any issues with that.