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## Re: Another puzzler

Most of these riddles seem to be about writing computer programs than about finding a solution with nothing but your brain (like #7). Others riddles seem to involve an impossible amount of work with pen and paper, but are actually not that much of it (like #2... fibonacci numbers become huge rather quickly). There are also a few which have a very easy solution which might not be that apparent (like #1).

I'm already proud of having found the way to solve #1 in 10 seconds (and performed it in a few minutes)

3*sum(1...(1000/3)) is the sum of all multiples of 3, 5*sum(1...(1000/5))is the sum of all multiples of 5. When adding them, we have the sum of all multiples of 3 or 5, but whose which are in both sums have been added up twice. These numbers are the multiples of 15 from 1 to 1000, so we have to deduct 15*sum(1...(1000/15)). This speeds up a lot by applying sum(1..n)= n*(n+1)/2. The result is 117835 if I'm not mistaken.

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## Re: Another puzzler

Good job Kris!
What it's about is the first site in the post.
http://projecteuler.net/

What is Project Euler?

Project Euler is a series of challenging mathematical/computer programming problems that will require more than just mathematical insights to solve. Although mathematics will help you arrive at elegant and efficient methods, the use of a computer and programming skills will be required to solve most problems.

The motivation for starting Project Euler, and its continuation, is to provide a platform for the inquiring mind to delve into unfamiliar areas and learn new concepts in a fun and recreational context.

Who are the problems aimed at?

The intended audience include students for whom the basic curriculum is not feeding their hunger to learn, adults whose background was not primarily mathematics but had an interest in things mathematical, and professionals who want to keep their problem solving and mathematics on the edge.

Can anyone solve the problems?

The problems range in difficulty and for many the experience is inductive chain learning. That is, by solving one problem it will expose you to a new concept that allows you to undertake a previously inaccessible problem. So the determined participant will slowly but surely work his/her way through every problem.

What next?

In order to track your progress it is necessary to setup an account and have Cookies enabled. If you already have an account then Login, otherwise please Register - it's completely free!
However, as the problems are challenging then you may wish to view the Problems before registering.

"Project Euler exists to encourage, challenge, and develop the skills and enjoyment of anyone with an interest in the fascinating world of mathematics."

Just saying.

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