One simple and common way to structure a Problem-solution essay is shown here:

Two other types of structure, block and chain, are shown in the diagram below. This is for a short essay, which includes the ‘situation’ in the introduction and ‘evaluation’ in the conclusion. A longer essay, for example one of around 1,000 words, with citations, would probably have these two sections as separate paragraphs in the main body.

Block Chain

 
Introduction
(including ‘situation’)

Problem 1

Problem 2

Transition sentence/paragraph

Solution 1

Solution 2

Conclusion
(including ‘evaluation’)
Introduction
(including ‘situation’)

Problem 1 &
Solution to Problem 1

Problem 2 &
Solution to Problem 2

Problem 3 &
Solution to Problem 3

Conclusion
(including ‘evaluation’)