WTF was up with the decay question about radium? I swear there wasn't even enough information to complete the answer! That test was dodgy...
IS THIS HOW YOU DO IT??? This is how I did it and no idea if it is right (this is from memory, the question is down in the car and I'm too lazy to go down there and get it
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I originally thought there wasn't enough info too, but because you are never given an amount, I just let Q=Q. The question tells you that A is a constant and equal to the initial amount, therefore A=Q {Q=Ae^(-k x 0), Q=A}. You know that Q is halved at t=1600, so Q/2 = Qe^(-1600k). Now divide by Q and the Q's cancel out => 1/2 = e^(-1600k), so ln 0.5 divided by -1600 = k. I seem to remember it as 0.0004...something or other???
As for the next part, I let the safe level of radium = x, so the current (and initial) level = 3x. I then said that after 1600 years, this level will have reduced to 1.5x, and now 1.5x = 3x e^(-1600k), and from here, I don't remember what I did, but I clearly remember ending up with the answer 2666.7 years, after dividing something by 1.5, and adding 1600 to something else.
Apart from that, it was a rather difficult test! Questions 1-4 were fairly easy, 7 was crap and time was up at the end of 8b). Eh. I tried, and it seems that many people agree it was tough!
