How much Uranium-235 would be left if the fossil were approximately 1.4 billion years old? in mg

Question

How much Uranium-235 would be left if the fossil were approximately 1.4 billion years old? in mg

Answer 1

That depends on how much (in mg) that you had to start with.

The half life of U-235 is about 700,000,000 years.  That means that every 700,000,000 years on half of the original amount has decayed.

Dividing 1,400,000,000 years by 700,000,000 years gives 2 half lives.  

So...in 700,000,000 years, you have half the original amount left.  After 1.4 billion years, you have half of the half of the original amount left, or one-quarter.

So...just multiply that by the original amount in milligrams and you have the remaining amount in milligrams.