See diagram in attache file.
The figure to the left is a voltage source, which has a internal resistance Ri.
The figure to the right is a resistor ( R ) (outside the source)
1. What is the maximum power that can be delivered to the load R?
2. What value of R achieves that?
It finds the maximum power delivered to load from a battery with an internal resiistance. The solution is detailed and well presented.