4. In a pond, there are 25 hungry pikes. For one pike to be satiated, it needs to eat three other pikes. What is the maximum number of pikes that can remain in the pond, with all of them being full?
Solution. The smallest number of creatures that can be eaten by three hungry creatures is five. In this way, five small creatures and four hungry creatures remain. Of the four hungry creatures, if one eats another and is then eaten, one small creature will remain. Therefore, six small creatures remain: To have at least seven small creatures remaining, each of them would have to eat three creatures, meaning a total of 21 creatures would need to be eaten, which would require at least $21+7=28$ creatures at the beginning, but this is not the case. Therefore, the maximum number of small creatures that can remain in the pond is six. $\cdot$

## VII Section