According to the ratio of 3: 4: 5, to put it bluntly, there are 3 trees planted at level 4 and 5 trees planted at level 6. In other words, if the fifth grade is omitted, the ratio of the fourth and sixth grades is 3: 5. In proportion, the two items before and after a ratio increase or decrease at the same time, and the value of the same multiple ratio remains unchanged. Therefore, when the latter item is 24 times more than the former item, the actual number of trees planted should be 36:60, which means that the former item and the latter item with a ratio of 3: 5 are increased by 12 times at the same time. So the final result is to increase each item in the ratio of 3: 4: 5 by 12 times at the same time, and the actual result ratio of trees planted in the last three grades is 36: 48: 60. That is to say, the number of trees planted in the three grades is 36, 48 and 60 respectively, making a total of 144.