God, it's not too hard to solve the math problem. Otherwise, I have to regret for not studying hard!

the fourth level is less and wider. When the area and volume are known, reverse the length of one side and the diameter. It's also three times, five times and two times, so as to pass the customs smoothly.

the fifth level, commercial work, earth rock engineering, volume calculation, is just a pediatrics.

the sixth level, equilibrium, is to use the previous calculation method to make an average distribution. There is nothing difficult.

the seventh level, the problem of surplus and deficiency, that is, the problem of double ways of thinking,...