Nuclear Chemistry Question, 10 easy points!?

Question

Please help me with this question, I don't understand it at all.

"The element selenium occurs in crops in certain regions of the country and is toxic to animals. The radioisotope Selenium-75 has a half-life of 120 days and is readily taken up by plants. If a cow ingested 0.1 milligrams of radioactive selenium by eating a plant containing the isotope on January 1, 1996, in what year will thee be less than 1µg (1x10 to the -6 power g) in the cow, assuming that none of it is excreted?

Answer 1

0.1 mg x 1 g/1000 mg=1x10^-4 g

1x10^-6 g=1x10^-4 g*0.5^(t/120 days)

1x10^-6/1x10^-4=0.5^(t/10 days)

0.01=0.5^(t/120 days)

ln(0.01)/ln(0.5)=t/120 days

t=(120 days*ln(0.01))/ln(0.5)

=797 days=2 years and 67 days

Answer: In March, 1998