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#### karush

##### Well-known member

- Jan 31, 2012

- 2,715

**40 grams**of radioactive substance decomposes to

**20 grams**in

**2 years**, then to the nearest gram the amount left after

**3 years**is

well i used the

**$N(t)=N_{0}e^{-kt}$**

So

**$20=40e^{-k2}$**thus deriving

**k=.3466**

Thus

**$N(3) = 40e^{-.3466(3)}$**resulting in:

**$N(3)= 14.1410$**or approx

**$14g$**

just seeing if this correct....thnx much