Carbon-14 is a radioactive isotope of carbon that is used to date fossils. There are about 1.5 atoms of cafbon-14 for every trillion atoms of carbon in the atmosphere, known as 1.5 ppt (parts per trillion). Carbon in a living organism has the same concentration as carbon-14. When an organism dies, the carbon-14 content decays at a rate of 11.4% per millennium (1000 years). Write an equation for carbon-14 concentration (in ppt) as a function of time (in millennia) and determine how old a fossil must be that has a measured concentration of 0.3 ppt.