`600599598596`

The first thing to do is to identify the sequence.Start by assuming the highest number has 1 digit:

6,0,0,5,9,9,5,9,8,5,9,6Does that work? No - because you have two zeros together.

So try with 2 digits:

60,05,99,59,85,96Does that work? No - 99 is larger than 05

Next three digits:

600,599,598,596Dos that work? Yes. Each of the values is less than the previous one, and most of them have a difference of one.

What number is missing? 597

You can to that manually just like that, and your task is to automate the process - but that's just the same as doing it manually, but in code. All you need to do is find the right number length by extracting each set of characters and converting that to a number. You can then look at the number sequences and see what you have, and if it's a descending sequence.

Pick a language from the 5 very different ones you tagged, and give it a go: it's not that complex when you start by doing it manually first to work out how.