For the purpose of the riddle, it takes place in a world where 10 bits (1000 satoshi) will buy you a beer.
Will you find the answer ? Is there any answer to this problem at all ? Let's try !
There is a marine boat coming back from a long time sailing on the sea.
To celebrate, 3 friends decided to go have a good, fresh beer in a pub.
They enjoyed it, and had a great time. At some point, one of them asked the waiter for the bill.
The waiter brought it, and the bill was 30 bits. It's 10 bits for a beer, so each of them gave 10 bits.
But, the pub manager said to the waiter that they're marines, they're good guys helping the country, and gave a 5 bits discount on the bill.
The waiter brought 5 bits back, but it can't be split evenly among the 3 of them so they decided to get 1 bit back for each of them, and leave 2 bits as a tip to the waiter.
So .. If each of them paid 10 bits and got 1 back, one beer costed 9 bits.
3*9 = 27 bits.
Plus the waiter's tip :
27 + 2 = 29 bits.
Where did the last bit go ?!
Enjoy
instead of 27 - 2 = 25 
