When a commodity with a unit purchase price of 40 yuan is sold at 50 yuan, 500 pieces can be sold. It is known that for every 1 yuan increase in the price of the commodity, the sales volume will decrease by 10 pieces. In order to make 8000 yuan profit, we should purchase () A. 400 B. 200 C. 400 or 200 D. 600

When a commodity with a unit purchase price of 40 yuan is sold at 50 yuan, 500 pieces can be sold. It is known that for every 1 yuan increase in the price of the commodity, the sales volume will decrease by 10 pieces. In order to make 8000 yuan profit, we should purchase () A. 400 B. 200 C. 400 or 200 D. 600

Suppose that the selling price is x yuan / piece, according to the meaning of the question, we can get [500-10 (x-50)] · (x-40) = 8000, ｜ x2-140x + 4800 = 0, ｜ x = 60 or x = 80, when x = 60, 500-10 (x-50) = 400, when x = 80, 500-10 (x-50) = 200, we should purchase 400 or 200 pieces

When a shopping mall sells a certain kind of clothing with a purchase price of 40 yuan at 50 yuan, 300 sets of clothing can be sold every day, and the sales volume will increase by 1 yuan
If the selling price is set at x yuan, please work out the functional expression between the daily sales profit y and the selling price,
If the selling price is set at x yuan, please work out the functional expression between the daily sales profit y and the selling price.

y=(x-40)*[300-5*(x-50)]
y=(x-40)*[300-(5x-250)]
y=(x-40)*(300-5x+75000)
y=(x-40)*(-5x+75300)
y=-5x^+75300x+200x-301200
y=-5x^+75500x-301200
*On behalf of the multiply sign
^Represents square
I made it myself,

When a shopping mall sells a certain kind of clothes with a purchase price of 40 yuan at 50 yuan, 300 sets of clothes can be sold every day. Every time the selling price of this kind of clothes is increased by 1 yuan, the sales volume will be reduced by 5 sets. If the shopping mall sets the selling price as X, please write down the functional relationship between the number of pieces of this kind of clothes sold every day and the value range of the level independent variable x

If the selling price is set at x yuan / set (x ≥ 50), the selling price of each set will be increased by (x-50), and the sales volume will be reduced by 5 (x-50). The actual daily sales volume is [300-5 (x-50)]
y=300-5x+250
=-5x+550
y=-5x+550
Because y > 0, X ≥ 50
So there are:
-5x + 550 > 0 and X ≥ 50
X < 110 and X ≥ 50
Therefore, the value range of independent variable x is: 50 ≤ x < 110

If the down jacket is sold at 50% off the marked price, it will earn 50 yuan. If it is sold at 60% off the marked price, it will earn 80 yuan
How much is the price and cost? How much discount can we get at most to make a profit of no less than 20 yuan?

What do you want to ask? Capital? It's very simple
You assume the price is X
0.5x-50 = 0.6x-80, price = 300
The cost is simple
0.5 × 300-50 = 100, that's the cost
How much discount does it take to make a profit of 20
Assuming a discount of Y
300Y=100+20
Y=4
At least 40% off