Okay, let's say you and your friend want to go to the amusement park, but you only have one ticket. You both really want to go, so how do you decide who gets to use the ticket?
Well, one way to figure it out is to use something called Lagrangian duality. Basically, you can think of it as a way to help you make decisions when there are limited resources.
Here's how it works: you start by defining what's called a "dual problem," which is another problem related to your original problem. In our amusement park example, the dual problem might be something like: how much would your friend be willing to pay for the ticket?
Using Lagrangian duality, you can then find what's called the "dual solution," which is the answer to the dual problem. So in our example, let's say your friend is willing to pay $10 for the ticket.
Now, you can use the dual solution to help you make a decision about the original problem. One way to do this is to use what's called the "Lagrange multiplier," which is a fancy term for a number that helps you balance the two problems.
In our amusement park example, the Lagrange multiplier might be something like: you'll let your friend use the ticket if they're willing to pay $5 or more. If they're only willing to pay $4, then you'll use the ticket yourself.
By using Lagrangian duality, you can come up with a fair way to make decisions when there are limited resources. And that's the basics of Lagrangian duality, explained like you're five!