The monthly sales volume is 500-(55-50)* 10=450 pieces.

The sales amount is 55 * 450 = 24,750 yuan.

Gross profit is 24750-40*450= 18000 yuan.

2. Resolution function between y and x.

y=(50+x-40)*(500- 10x)

3. When purchasing 10000 yuan, the sales unit price is set to 50+x yuan.

8000=(50+x-40)*(500- 10x)

The solution of X 1= 10; X2=30

When X 1= 10, the cost = 40 * (500-10 *10) =16000 yuan, and sometimes the cost exceeds 10000 yuan, it will be discarded.

When X2=30, the cost = 40 * (500-30 *10) = 8,000 yuan, which is in line with the question.

So the selling unit price should be set at 50+30=80 yuan.

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