Relationship between object distance, image distance, focal distance for doubly convex lenses

[j_limb14]

Problem:

So, I finished part A of this question, but I need help with finishing part B.

 

[Harry_the_cat]

\(\displaystyle \frac{1}{a}+\frac{1}{b} \neq \frac{1}{a+b}\)
You add fractions by first getting a common denominator.