Solution: Suppose a * * * buys X bicycles.
The equation is 400x=500 times (5/6x-4).
The solution is x= 120.
Profit = 120 times (500-400)= 12000 yuan.
Solution: If the number of bicycles purchased by a * * * is X, the title says that when the number of bicycles sold is 5/6, it is equal to the cost, and plainclothes will use X to find out the cost. Because there are 400 bicycles, 400x is the cost. As can be seen from the title, the price of bicycles is 500, which is exactly the cost of selling 5/6-4 bicycles, so it is 500 times (5/6 X= 120, because the price of each car is more than the cost price 100, and there are 120 vehicles, so the profit is: self-walking.
I hope I can help you.