A student puts a $30\,\text{cm}$ ruler on a fulcrum positioned at the $15\,\text{cm}$ mark. She next adds a $50\,\text{g}$ mass at the $23\,\text{cm}$ mark and a $20\,\text{g}$ mass at the $11\,\text{cm}$ mark, as shown. Which mass should she add at the $7\,\text{cm}$ mark in order to make the ruler balance again?
- A$30\,\text{g}$
- B$40\,\text{g}$
- C$47\,\text{g}$
- D$133\,\text{g}$