Calculus
posted by Brandon .
The halflife of radioactive strontium90 is approximately 31 years. In 1960, radioactive strontium90 was released into the atmosphere during testing of nuclear weapons, and was absorbed into people's bones. How many years does it take until only 11 percent of the original is left?

.11=e^(ln2*years/31)
take the ln of each side..
ln(.11)=ln2 * years/31
years= 31*ln(0.11)/ln(2)= ????