680 g of sodium 24 was placed in a container to decay if there is now 85 g of sodium 24 left how long has the sodium been decaying

Explanation:

The time taken for the radioactivity of a specified isotope to fall to half its original value is called half life of that radioactive material.

Half life of sodium-24 is hours.

Initial amount of sodium-24 is

In hours,the quantity falls to

In another hours,the quantity falls to

In another hours,the quantity falls to

So,it takes hours for the quantity to become .

i'm assuming that the correct answer is indeed the first one/ "a"..

a 20-cm-long, cylindrical graphite (pure carbon) rod is inserted into an oxidizing atmosphere at 1145 k and 1.013 x 105 pa pressure. the oxidizing process is limited by the diffu- sion of oxygen counterflow to the carbon monoxide that is formed on the cylindrical surface.