Question 657390
The half-life of a radioactive element is the amount of time it takes for half of the atoms to decay.  
This means that if we start with 1 gram of an element, after one half-life there will be 0.5 grams left.
Assuming the half-life of Carbon-14 is 5760 years, if we start with 2 grams then
after t=5760 years (one half-life), there will be 1 gram left.
Since with each half-life we lose half of the remaining amount, if t=11520 years this is equal to two half-lives, so there will be 0.5 grams left.
In general, we can write N = N0(1/2)^n where N0  is the initial amount and n is the number of half-lives
N0 = 2 grams in this case
t=23040 years is four half-lives, so the amount remaining is 2(1/2)^4 = 0.125 grams.