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# Core 3 Q5 Jan 2009 OCR watch

1. I can do this question easily enough but the wording implies I am missing something obvious. Here's the question.

The mass, M grams, of a certain substance is increasing exponentially so that, at time t hours, the mass is given by M=40 e ^(kt)
where k is a constant.
The following table shows certain values of t and M.

The table gives M=80 when t=21 and blanks for M when t = 0 and t = 63

The question is then
(i) In either order,

(a) find missing value in table (M for t=0 and t=63)

(b) find k.

My question is, although you can find M(0) easily enough, how could you find M(63) without finding k first? The wording of the question implies that this is possible.
2. K is a constant, and you are given two values (M=80 when T=21).

Put in 80=40e^21k

Divide both sides by 40

2 = e^21k

Take natural log of both sides

ln(2)=21k
ln(2)/21 = k

Then solve for rest of questions.
3. By the way, does it help you if I say that 63 is 3x21?
4. I also think that they specifically say in either order, so it doesn't matter.
5. Ah, answered my own question. Find M(0)=40 gives us the fact that increasing t by 21 doubles the value of M. Hence we could have M(42)=160 and then M(63)=320
6. (Original post by Kolasinac138)
I also think that they specifically say in either order, so it doesn't matter.
True, but if they say in either order then it is worth thinking that there is something a little clever to be thought of. It gives little entertainment when doing these questions.

BTW, +1 to the person whos suggested the relationship above.
7. Thanks ... Kolasinac

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