Solve for F given the information and the diagram, and describe how to locate the image for an object in front of a concave mirror between C and F.

The meter rule is uniform and balanced at its center, so the pivot is at the $50 \text{ cm}$ mark.  
A force of $100 \text{ N}$ acts at the $20 \text{ cm}$ mark. Its distance from the pivot is $50 \text{ cm} - 20 \text{ cm} = 30 \text{ cm}$. This creates an anticlockwise moment.  
An unknown force F acts at the $70 \text{ cm}$ mark. Its distance from the pivot is $70 \text{ cm} - 50 \text{ cm} = 20 \text{ cm}$. This creates a clockwise moment.

For the rule to be in equilibrium, the sum of clockwise moments must equal the sum of anticlockwise moments about the pivot.
$$
\text{Anticlockwise Moment} = \text{Clockwise Moment}
$$
$$
100 \text{ N} \times 30 \text{ cm} = F \times 20 \text{ cm}
$$

$$
3000 \text{ N cm} = F \times 20 \text{ cm}
$$
$$
F = \frac{3000 \text{ N cm}}{20 \text{ cm}}
$$
$$
F = \boxed{150 \text{ N}}
$$