To level up, he needs 60 experience points, but he can only gain 80% of the experience from neutral creeps. Therefore, he needs to kill a creep that provides 75 experience points.
However, a Level 4 turtle provides 85 experience points. After providing the hero with the necessary experience to level up, there are 10 experience points left over. At this point, the hero has reached Level 2, and the experience rate gained from neutral creeps decreases to 70%, meaning the hero can only gain 7 experience points.
The calculation is as follows: 75 * 0.8 + 10 * 0.7 = 67.
From this, we can conclude that lower-level heroes gain more experience when fighting neutral creeps. A trick arises from this: since consuming an Experience Book grants a fixed value of 150 experience points, leveling up by killing neutral creeps first and then consuming the book allows the hero to gain relatively more experience.