Posted by **koteswaramma** on Friday, August 15, 2014 at 9:22am.

a radioactive substance decays 10% in 5 days. The amount remains after 20 days?

- chemistry -
**DrBob222**, Friday, August 15, 2014 at 2:20pm
You need to determine k, the rate constant and I would do it this way.

ln(No/N) = kt

No = 10

N = 90

k = solve for this

t = 5 days.

Then knowing k, redo

ln(No/N) = kt

No = 100

N = Solve for this.

k from above

t = 20 days

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