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It's not profitable. Unless you have really cheap/free electricity it's not very profitable right now. How ever the coin value may increase in the future. The reason many people do it is because you are directly contributing to science. Such as finding cures for the worlds deadliest diseases, the search for ET, or other scientific problems that require computing power. With Gridcoin you will get paid back some for your contributions.

Ill bet on the coin price rising. How many coins can you mine in a day?

On a phone depending on which science project you choose you can mine about 2 coins a day depending on your phone hardware as well. Of course newer high end hardware will mine more. Also with PC if you have graphics cards and CPU you can get quite a bit. Maybe 10 a day or so.

Wow nice! Will these coins be worth much in the future? Because i would just hold these things for a long time

They could be. As with any crypto it is still basically a form of gambling vs investing.

For me its right on the border of break even, but Gridcoin price intermitently pumps and if you hold and sell on the highs, then it really is profitable.
For me, I just take the BTC, wait for GRC to drop back and then just buy an even bigger stake.
Because Gridcoin is run by people who are passionate about science, I believe it will last much longer than a lot of alts that are just a way to gamble on the crypto-exchanges.