The percentage is changing quite regularly. The algorithm they are using is described here: https://minexcoin.com/html/download/wpeng.pdf (starting on page 7 of the PDF).
And even though in the "real" economy 70% is absolutely unrealistic, I guess no one would be surprised to make 70% in a year in cryptocurrencies. And since you don't have to transfer the coins out of your wallet I'm not seeing a risk here. I still could sell my coins in the meanwhile if I wanted to (and then don't get the interest).
(But hey, what do I know, I'm just a newbie. I just tried a monthly parking and everything worked out as promised.)

You can park your https://minexcoin.com/ at the Minex bank. Current rates are:
0,32% for daily parking
2,9% for weekly parking
15,4% for monthly parking
70% for yearly parking.

Nice thing about it: you can leave your coins in your wallet and don't have to transfer them somewhere else.

Announcement thread: https://bitcointalk.org/index.php?topic=1847292.13160

If you are looking for the people behind this project, look at the team-page: https://minexcoin.com/?r=site/team

Your calculations are wrong. With 16.1% interest rate you gain after four months ~81.2% and with a 18.4% interest rate ~96.5%.
Beside that: "only 180%" are way more than doubling, nearly tripling.

Where exactly do you have problems?
You need to sign up (https://minexbank.com/sign-up) with a Minexcoin address of yours which contains at least 0,1 Minexcoins (the minimum which you can park), after that you specify how many coins you want to park and for how long and then press the "create" button. That's it.

At the moment even the developers recommend not to use it:

I wasn't aware that there is a copycat around. I just searched the fees-page on coinexchange for "minex" and couldn't believe what I was seeing. Thanks for clearing this up.

Withdrawal fee on coinexchange.io is really 1 Minexcoin? That seems quite pricy in comparison to Livecoin where the fee is 0.001 MNX.

I tried it with Chrome (which worked yesterday) and Firefox (both on Linux) and both don't show the Google captcha at the moment. I'll give it later a try again.

Am I the only one who is not shown a capture at the faucet (http://eu.minexpool.nl/tbs) at the moment?

Why not set a bookmark? Beside that: the forum offers you a watchlist (upper left corner) which you could use.

Thanks a lot. Again something new learned.

Where do you have this number from?

Thanks for your reply but unfortunatly still not what I'm looking for.

I'm searching a portfolio manager to keep track of my trades but I'm not able to find what I'm looking for. I know there are tons of products out there (e.g. https://www.producthunt.com/ask/3620-what-s-the-best-cryptocurrency-portfolio-tracker) but they are all web-based or for Mac. What I had in mind:
a desktop app which can run on Linux, free of cost, preferably open source. I don't mind to enter transaction data manually. I also don't need realtime quotes (so a few minutes delay is totally fine), fancy diagrams or other bells and whistles.
I just want to keep track of my trades because of tax-issues and to have an overview what the daily percentage change is (since livecoin.net does not show me that information for whatever kind of reasons).
Is anyone aware of such an application and I'm just too stupid to find it or is everybody using a webbased (/mobile) solution?

The calculation of the activity is not a black box:

@Deathwing: The output of the faucet is not formatted correctly. The following message was displayed to me just right now: "You have been awarded with 5.483E-5 Minexcoin! "
Beside that: the link to the FAQ (http://eu.minexpool.nl/faq) is not working. I always get an error stating "Cannot GET /faq".

I'm on a Linux system. Just checked with Chrome which also does not work.

Same here with Firefox 57.0.1