## orbie 3 years ago The mass m(t) remaining after t years from a 90-gram sample of a radioactive element is given by m(t)=90e^-0.3t. When will the mass remaining equal 9 grams? Round your answer to the nearest hundredth of a year.

$9=90e^{-.3t}$ we solve for $$t$$ divide by 90 get $\frac{1}{10}=e^{-.3t}$ or if you prefer $.1=e^{-.3t}$ then in log form this means $\ln(.1)=-.3t$
finally $t=-\frac{\ln(.1)}{.3}$and then a calculator