An O(N^2) time complexity algorithm will timeout, opt for an O(N) time complexity algorithm.

We can forget about 1-5 and 16-19 because they are inside 1-6 an 10-20. 1-6 and 5-11 = 1-11. And 1-11 and 10-20 = 1-20. It's length = 20-1 = 19;

You're right, everything overlaps, so you end with a unique interval [1, 20], which length is 19.

all 3 others overlap each other so it's like [1, 11], which gives 10 as well.

But you already used number 10 in the previous intervals (so, only 9 for those)

The representation of a number as floating point or strings is exactly that: a representation. A string can exactly represent the input and output, so that's what's used. What you want to do with them is up to you.

You should add the numbers. That's it.

There's no ambiguity in the input or output. How you would implement it is your task.