The half-life of rubidium-89 is 15 minutes. if the initial mass of the isotope is 250 grams, how many grams will be left after 100 minutes?
mathematics

Exponential decay can be expressed as:

f=ir^t, f=final value, i=initial value, r=common ratio or "rate" and t=times ratio is applied or "time"

We are told that the half life is 15 minutes and that means that i=2f at that point

1=2r^15

1/2=r^15  if we raise each side to the power of 1/15 we have:

(1/2)^(1/15)=r  now that we know the rate and the initial value, 250g we have:

f=250(1/2)^(1/15)^t which is equal to:

f=250(1/2)^(t/15)  so when t=100 minutes...

f=250(1/2)^(100/15) g

f≈2.46 g  (to nearest hundredth of a gram)