half life av radium 225=15 daes
10 grams/26 grams=0.384615384
dekae go as e-kt
half life...dekae tu 1/2, so e-kt=1/2
need tu find exponent av e tu get 0.384615
ln(e^-kt)=-kt=ln(0.384615)=-0.9555
ln(1/2)=0.693147
0.9555/0.693147=1.378 half lifes
1.378*15=20.7 daes