00:01
So in this problem, we're working with a radioactive substance called strontium 90.
00:10
And we're told that it follows this exponential decay formula.
00:16
A of t, the amount after time t, is equal to the initial amount e to negative 0 .0244t, where t is in years.
00:33
We're told that a scientist initially has 400 grams.
00:41
We're first asked, what's the decay rate? all right, so to find the decay rate, we've got to look at this exponential right here.
00:57
So e to the negative 0 .044 is what? we'll go to our calculator.
01:05
0244 negative, take e to that number, and that is 0 .0244 is 0 .0244 is 0 .4 .244 .5.
01:11
0 .9759.
01:18
So to find the decay rate, we go 1 minus this number, 0 .9759, and that gives us a decay rate of 0 .0241.
01:35
So that means 2 .41 percent.
01:41
Decay.
01:42
There's our decay rate.
01:45
Okay.
01:46
Now then, we're next asked, after 10 years, how much does he have left? so a of 10 is 400 e to the minus 0 .0244 times 10.
02:11
So 0244 times 10 negative, e to that exponent times 400 is 313 .4 grams.
02:30
That out.
02:34
All right.
02:35
Then we're next asked, how long till we get to 100 grams? all right, so we set it up, right? so it's 100 grams.
02:48
It's 400 grams, because we started at 400, e to the amount of 0 .044t...