Dear Sir

I would be grateful if you can help me to solve the the below questions

I managed to get the answer to part (a) but cannot get the time 11.16 minutes in part b

thanks

Kingsman

I

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- Sep 21st 2010, 04:39 AM #1

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- Sep 22nd 2010, 03:06 AM #2

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This is a seperable differential equation with solution:

$\displaystyle x=10+Ae^{-t/50}$

As $\displaystyle x(0)=20$, we have $\displaystyle A=10$

so:

$\displaystyle x=10(1+e^{-t/50})$

To find when $\displaystyle x(t)=18$ we solve:

$\displaystyle 10+Ae^{-t/50}=18$

which has solution:

$\displaystyle t=-50 \ln(0.8)=11.16$ minutes (rounded to two places of decimal)

CB