Maths help please!

The amount of a certain substance at time t (measured in seconds) is given by N(0.850t), where N is some constant. How long (in seconds) does it take until only 5% of the starting amount remains?

Can anyone help me with this question please?

Reply 1

Zacken

Original post by elef95

Uh, surely the starting amount would be

t = 0 \Rightarrow N(0) = 0

. Do you want to fix your question or post up a picture of it? (The latter would be very much welcome!)

Reply 2

nerak99

Original post by Zacken

Uh, surely the starting amount would be

t = 0 \Rightarrow N(0) = 0

. Do you want to fix your question or post up a picture of it? (The latter would be very much welcome!)

Could this be a Q where the stuff is used up and so the initial condition may be that the amount is non zero.

Post the question and all will be clear.

(edited 8 years ago)

Reply 3

Zacken

Original post by nerak99

This is a Q where the stuff is used up and so the initial condition may be that the amount is non zero.

Post the question and all will be clear.

Yes, that was my point. :tongue:

Reply 4

elef95

Original post by Zacken

Uh, surely the starting amount would be

t = 0 \Rightarrow N(0) = 0

. Do you want to fix your question or post up a picture of it? (The latter would be very much welcome!)

Original post by nerak99

Could this be a Q where the stuff is used up and so the initial condition may be that the amount is non zero.

Post the question and all will be clear.

Thanks for the replies! That's the whole question.

Reply 5

elef95

Original post by Zacken

Yes, that was my point. :tongue:

Do you have any idea how to answer this question?

Reply 6

Zacken

Original post by elef95

Do you have any idea how to answer this question?

As it is written, the question doesn't make sense. :confused:

Could you provide a picture? :smile:

Reply 7

username1560589

Original post by elef95

The amount increases with time, so the only possible solution is at 0.

Reply 8

elef95

I attached the image. Why can't the amount decrease over time?

Original post by Zacken

As it is written, the question doesn't make sense. :confused:

Could you provide a picture? :smile:

Original post by morgan8002

The amount increases with time, so the only possible solution is at 0.

ah.jpg379.7KB

Reply 9

username1560589

Original post by elef95

I attached the image. Why can't the amount decrease over time?

The question is

N(0.580^t)

not N(0.850t), as you said earlier.

Work out the initial amount. Then set the ratio of the amount at some later time to the initial amount to be 5%.

Reply 10

elef95

I'm sorry I wrote the question incorrectly! Do you know how to answer it now?

Original post by Zacken

As it is written, the question doesn't make sense. :confused:

Could you provide a picture? :smile:

Reply 11

Zacken

Original post by elef95

I'm sorry I wrote the question incorrectly! Do you know how to answer it now?

Initial amount is when

t = 0

and the initial amount is then

N

. When there is only 5% remaining, we want to solve:

\displaystyle \frac{5N}{100} = N \times 0.58^t \Rightarrow 0.58^t = \frac{5}{100}

then use logarithms to solve this, can you take it from here?

PS: Can you now see the importance of attaching a picture or link of the question? :smile:

Reply 12

elef95

Original post by Zacken

Initial amount is when

t = 0

and the initial amount is then

N

. When there is only 5% remaining, we want to solve:

\displaystyle \frac{5N}{100} = N \times 0.58^t \Rightarrow 0.58^t = \frac{5}{100}

then use logarithms to solve this, can you take it from here?

PS: Can you now see the importance of attaching a picture or link of the question? :smile:

Thank you! I got the answer :smile: