HL 2011 Q 4(c)
Have solution here showing
then turns to
Can someone please explain as to where k and 0.5 come from?
The part where you mentioned Ae^(-b(t + k)) = 0.5(Ae^-bt) refers to Q 4(d). Maybe that was the thing that was confusing you. Q 4(d) is not part of Q 4(c) (i.e. you're not using the same equation for the two questions).