m=ae-kt, where m is the mass after t years, a is the initial mass when t=0 and k is the decay constant.
If 15 years is the half-life then when t=15, m=a/2, so a/2=ae-15k, e-15k=½.
-15k=-ln(2), k=ln(2)/15, so m=ae-(t/15)ln(2), which is the same as m=a×2-t/15.
When a=150mg and t=2040-2000=40, m=150×2-40/15=150×2-8/3=23.62mg.
(Quick check: If t=15 years, m would be 150/2=75mg; if t=30 years, m would be 75/2=37.5mg; if t=45 years, m would be 37.5/2=18.75mg. 40 years is between 30 and 45, and 23.62mg is between 18.75mg and 37.5mg.)