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)