Lead-209, a radioactive isotope, decays to nonradioactive lead over time. the half-life of lead-209 is 8 days. suppose that 20 milligrams of lead-209 are created by a particle physics experiment. write an equation for the amount of lead-209 present t days after the experiment

Radioactive elements undergoes first order decay. 

Now, for 1st order decay kinetics, rate constant (k) can be estimated as follows

k = 0.639/ t1/2,
where t 1/2 = half-life of radioactive material = 8 days (For Pb-209)

Also, we know that
  .................(1)

Here, Co = initial conc. of radioactive element
Ct = conc. of radioactive element at time t
t = time required to reach the concentration Ct

Thus, equation 1 can be used to estimate the amount of lead-209 present after 't' days.

0.50 hope this  

The answer to your question really depends on three things: how much water you have, how cold it is to start out, and how cold the things around it are. water actually freezes when it gets to 32 degrees fahrenheit (0 degrees celsius), but the time it takes to get there may be different.  let's start with the first. if you take two glasses, and fill one with a tiny bit of water, and the other about halfway, then put them both in the freezer, the one with less water will freeze first (you can try this at home, but i recommend using plastic cups and not glass ones).  now let's move on to the second part. let's say you have two glasses, and you fill one with really cold water that has been in the refrigerator, and the other with really hot water from the sink. if you put both of them in the freezer, the one that started out colder will freeze first.  for the third part, let's imagine that you have two glasses with the same amount of water in them, and the water is at the same temperature. imagine putting one outside on a really really cold day in georgia, and having a friend in alaska put one outside on the same day. since it would be so much colder in alaska, the glass of water there would freeze before yours.  so, if you took a tiny bit of really cold water in a glass, and put it outside on a cold day in alaska, it would freeze a lot faster than a big glass of hot water outside on a cold day in georgia.