[Maude79]

The half-life of 40K (potassium 40) is 1.25 billion years.?
If a rock that formed just before the beginning of the Proterozoic Eon (2,500,000,000 years ago) originally contained 200gm of 40K, about how much would be left in the rock today?

[UG]

If a rock that formed just before the beginning of the Proterozoic Eon (2,500,000,000 years ago) originally contained 200gm of 40K, about how much would be left in the rock today?

How many half lives of 40-K is 2,500,000,000 years?
The answer to the above is how many times do you need to halve 200 g to get the answer.

[vmelkon]

Apply the standard equation
result = m * 0.5^time/half-life

result = 200 g * 0.25