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

I’d have to say no because without oxygen in the water the fish would all be dead. so if you want to fish for dead fish, pick the ponds with the lowest oxygen concentrations.