1. **State the problem:** Determine the focal length $f$ of a given lens to check if it can replace a damaged spectacle lens. 2. **Given:** Hypothesis focal length range is $8.0\text{ cm} \leq f \leq 12.0\text{ cm}$. 3. **Variables:** Independent - object distance $u$; Dependent - image distance $v$; Controlled - light intensity. 4. **Procedure summary:** Measure $v$ for given $u = 20,30,40,50,60,70 \text{ cm}$ by adjusting the screen to get a sharp image. 5. **Tabulation:** Calculate $uv$ and $(u+v)$ for each measurement. 6. **Graph:** Plot $uv$ (y-axis) against $(u+v)$ (x-axis); determine slope $S$ of the straight line. 7. **Focal length calculation:** According to the experiment, the focal length is equal to the slope, $$f = S$$. 8. **Explanation:** The relationship arises from the lens formula $$\frac{1}{f} = \frac{1}{u} + \frac{1}{v}$$ and algebraic manipulation leading to a linear relation between $uv$ and $u+v$ where the slope equals $f$. 9. **Possible errors:** Parallax error when reading $v$, drained cells reducing light intensity, and ambient light affecting sharp image detection. Thus, focal length $f$ is found from the slope $S$ of the $uv$ versus $(u+v)$ graph.