Assume a substance has a half-life of 11 years and the initial

amount is 144 grams.

How much remains at the end of 5 years?

How long will it be until only 30 % remains?

3 answers

If A(t)=quantity in grams left after t years,
A0 is the initial quantity in grams, then
R=rate of decay (0<R<1)

A(t)=A0 R^t
For a half life of 11 years,
t=11
A(t)=0.5A0
R^t=A(t)/A0=0.5
Take ln and put t=11
11ln(R)=0.5
Solve for R.

You can solve the second part similar to the first. Post your answers for checking if you wish.

thanks

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