An isotope of sodium, 24 na, has a half-life of 15 hours. a sample of this isotope has mass 2 g.
(a) find the amount remaining after 60 hours.
(b) find the amount remaining after t hours.
(c) estimate the amount remaining after 4 days (to the nearest thousandth).
(d) estimate the time (to the nearest tenth) required for the mass to be reduced to .01 g by graphing the function.

We know that, at 0hours, t = 0, the sample is 2g
notice, we're doing b) first, since, without the equation, you can't get a)
b)

thus 

notice, since the isotope is decaying, is a negative rate of change.

now.. hmm let's try to find "r"

well, we know it has a half-life of 15hours, so, 15hours later, the 2g will be come 1g, thus t = 15, A = 1



a)

well, simply set t = 60 to get A

c)

well, 4days is 96hours, so just set t = 96 to get A

d)

well, that simply requires you to graph it and check where A or A(t) becomes 0.01

you can always get it by using logarithms, but in this case, you're asked to get it from the graph.