If the half life of uranium 232 is 68.9 how much of a 150 gram sample is present after 250 years

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