the half life of C14 is 5730 years. if a sample of C14 has a mass of 20 micrograms at time t = 0, how much is left after 2000 years? I learned from somewhere that these exponential decay and half life problems use the equation y = ab^t or y = a(1+r)^t where y = total, a = initial amount, b or r = growth rate and t = time I think I have to solve for b, but I am not sure which t to use, whether its t=0 or t = 2000. I know a is given to be 20mg, but is y also a...so I am not really sure how to go about solving this. The end answer is 20exp (-ln/5730)(2000)...but I have no idea how to get to that. Please help, I can't even find the start in this situation because the y and a are confusing me.