A radioactive material, for example the isotope carbon-14, disintegrates at a rate proportional to the amount currently present. LetW(t) be the amount present at timet, measured in days, andrbe the proportionality constant, the decay rate.

(a) Write a differential equation to describe the decay process.

(b) If 100 mg of carbon-14 decays to 82.04 mg in one week, determine the decay rate,r.

(c) Find an expression for the amount of carbon-14 present at any time,t.

(d) Find the time required for the thorium to decay to one-half its original amount.

i know that the de :

dW/dt = -k dt

and W(t) = A e-kt

but i dunno how to solve (b), (c) and (d)....

pls help me....