A restrictive condition placed on one or more numerical or geometric variables.

The use of constraints is essential in solving linear optimization problems.

### Example

At least 150 litres of paint in total will be used to paint the classrooms in a school whose total surface area is less than 500 m². White and green paint can be used to match the school colours. According to the data provided by the decorator, the amount of green paint used should be at most 3 times the amount of white paint used. Given the characteristics of the paint, one litre of white paint covers 2 m² and costs $12, while one litre of green paint covers 3 m² and costs $15. How many litres of each colour should the decorator use to keep costs as low as possible?

The following constraints must be considered in this situation :

- at least 150 litres of paint,
- for a surface area of less than 500 m²,
- 3 times more green paint than white paint.