A superhero is rendered powerless when exposed to $40$ or more grams of a certain element. A $700$ year old rock that originally contained $250$ grams of this element was recently stolen from a rock museum by the$$ superhero's nemesis. The$$ half$-$life of the element is known to be $300$ years.

$$a$)$ How many grams of the element are still contained in the stolen$$ rock?

$$b$)$ For how many years can this rock be used by the$$ superhero's nemesis to render the superhero$$ powerless?