half life means is the amount of time only half the uranium is left. So after 68.9 years only 150/2 = 75 g is left. Another 68.9 years go by and then 75/2 = 37.5 g remains. See the pattern here?
then you can divide 250 by 68.9 to see how many times it's halved or just write the equation:
A = 150(0.5)^(t/68.9)
t is 250 years, A is amount left
A = 150(0.5)^(3.63)
A = 12.13 g
