4. Every day, from Monday to Friday, the old man went to the blue sea and cast his net into the water. Each day, the net caught no more fish than the previous day. In total, over the five days, the old man caught exactly 100 fish. What is the smallest total number of fish he could have caught on the three days - Monday, Wednesday, and Friday?
Answer: 50.

Solution. Estimation. On Tuesday and Thursday, the old man caught no more fish than on Monday and Wednesday, so over the specified three days, he caught no less than half of 100, which is no less than 50 fish.

Example. If the old man caught 25 fish each of the first four days and caught nothing on Friday, the conditions of the problem are met, and exactly 50 fish were caught over the specified three days.