SOLUTION: A 15 g sample of radioactive iodine decays in such a way that the mass remaining after t days is given by m(t) = 15e−0.087t, where m(t) is measured in grams. After how many

Algebra ->  Exponential-and-logarithmic-functions -> SOLUTION: A 15 g sample of radioactive iodine decays in such a way that the mass remaining after t days is given by m(t) = 15e−0.087t, where m(t) is measured in grams. After how many       Log On


   

Question 1072026: A 15 g sample of radioactive iodine decays in such a way that the mass remaining after t
days is given by m(t) = 15e−0.087t, where m(t) is measured in grams. After how many days are there
only 5 g remaining?
Found 2 solutions by ikleyn, josgarithmetic:
Answer by ikleyn(52776) About Me  (Show Source):
You can put this solution on YOUR website!
.
Do you think the mass goes to nowhere ?


If you think so, it is a big mistake.



Answer by josgarithmetic(39616) About Me  (Show Source):
You can put this solution on YOUR website!
m%28t%29=cross%2815e-0.087t%29
m%28t%29=15e%5E%28-0.087t%29------THIS way.

ln%28m%29=ln%2815%2Ae%5E%28-0.087t%29%29
ln%2815%29-0.087t=ln%28m%29
-0.087t=ln%28m%29-ln%2815%29
0.087t=ln%2815%29-ln%28m%29
highlight_green%28t=%28ln%2815%2Fm%29%29%2F0.087%29


m=5, evaluate t.
t=ln%2815%2F5%29%2F0.087
t=ln%283%29%2F0.087
highlight%28t=12.6%29 or about 12 and a half day