A uniform plank is supported by a pivot at its middle. Two children P and Q are seated on the plank at the positions indicated. Child P has a mass of $25\,\text{kg}$. The plank is in equilibrium. What is the mass of child Q?
- A$20\,\text{kg}$
- B$25\,\text{kg}$
- C$31\,\text{kg}$
- D$45\,\text{kg}$