# Advent of Code F# – Day 20

Yan Cui

I help clients go faster for less using serverless technologies.

The real-time data platform that empowers developers to build innovative products faster and more reliably than ever before.

The source code for this post (both Part 1 and Part 2) is available here and you can click here to see my solutions for the other Advent of Code challenges.

Description for today’s challenge is here.

Once you understood what this challenge is really asking the solution becomes really simple:

If you read the question carefully, what it’s really saying is:

For every house number N, only elves whose number M that is a divisor of N will visit this house and deliver M * 10 gifts. Find the smallest N where Sum(M * 10) >= 29000000.

So our first task is to find all divisors for any given number N, and since each divisor has a opposite (as in, for a divisor M, N/M is also a divisor) so you only need to iterate up to Sqrt(N).

And since my input is such a high number, we can safely start from a relatively high number for N too (100K in this case) and use Seq.unfold to generate an infinite sequence of numbers.

## Part 2

Part 2 adds a small twist to proceedings – only divisors M that are <= N/50 will be considered, and the multiplier is now 11 instead of 10.

Notice that since we’re not looking for ‘all’ divisors anymore, so I also took the liberty to rename the function to findDivisors so it more accurately describes what it does now.

Whenever you’re ready, here are 4 ways I can help you:

1. Production-Ready Serverless: Join 20+ AWS Heroes & Community Builders and 1000+ other students in levelling up your serverless game. This is your one-stop shop for quickly levelling up your serverless skills.
2. Do you want to know how to test serverless architectures with a fast dev & test loop? Check out my latest course, Testing Serverless Architectures and learn the smart way to test serverless.
3. I help clients launch product ideas, improve their development processes and upskill their teams. If you’d like to work together, then let’s get in touch.
4. Join my community on Discord, ask questions, and join the discussion on all things AWS and Serverless.