A uniform metre rule with mass $100\,\text{ g}$ is held up by a pivot at the $40\,\text{ cm}$ mark and by a string attached at the $100\,\text{ cm}$ mark. The string goes over a frictionless pulley and supports a mass of $20\,\text{ g}$, as shown in the diagram. At what mark on the rule should a $50\,\text{ g}$ mass be hung so that the rule is in balance?
- A$4\,\text{ cm}$
- B$36\,\text{ cm}$
- C$44\,\text{ cm}$
- D$64\,\text{ cm}$