I Have a Wisdom Backpack

Chenfeng has a wisdom backpack that will ask him strange questions every day. Simple question: A group of ducks passed downstream of the Menqian Bridge. What is the next sentence? Chen Feng smiled broadly. Intermediate question: There are four trees in the park. If two are cut down, how many people will be left? Chenfeng's smile solidified. Difficult question: L+1993=Q, what is the polarization period of Optimus Prime's windshield? Chenfeng's eyes widened. Mysterious question: Assume that the exit speed of the bullet is 250m/s, the coverage range of the level 3 mental power user's immobilization spell is 38m, and the action time is 2 seconds. How much mental power is used to immobilize the bullet? Chen Feng thought for a moment, let's answer the simple questions! I can get full marks on simple questions.