Is time measured in hours or minutes?
5000 = C e^(k15)
40000 = C e^(k60)
Divide the second by the first, side by side:
8 = e^(60k-15k)
8 = e^(45k)
ln 8 = 45k
k =ln8 /45
Plug into one of the conditions above to find C. C is the original population, because it is the y-value at t=0.