"Chongqing Yilin MLM case revealed that a primary school math problem was cheated by 60 million." What is the elementary school math problem mentioned in this case?

Yilin's lecturer once said during the training: Fast-growing poplar became a success in 7 years, with an investment of 3,980 yuan. If you buy 1 mu of fast-growing poplar trees that have been growing for two years, you can produce 15 cubic meters of wood in five years. 700 yuan per cubic meter, the profit per mu is 6520 yuan. If you invest less than 40,000 yuan and buy 10 mu of land, you will get a profit of more than 60,000 yuan in five years, and the rate of return will exceed 100%. This is the income conversion formula formulated by Zhao, the boss of Yilin. It is this simple math problem that primary school students can calculate, which makes many citizens feel as if they have touched the cash machine of the "green bank" and withdrawn a lot of money.