I would go for integration rather than doing that shitty method in the markscheme. Now I know CIE syl. doesn't have calculus,
but I've heard the examiners entertain new methods.
Q = Integral (I) dt
where I = I.e^-(lambda)t
where I. is the initial current (I at t=0)
to find lamda just substitute any value of t and corresponding I in the equation I = I.e^-(lambda)t
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If you're having trouble with the integration, go for the method suggested in the markscheme. Count the boxes, calculate the area of one box, and
find the total area. Pretty troublesome if you ask me
