10b+a=4(b+a)+9 ②

A=7, b=5, and the original number is 75.

2. If the original price is A and the current price is B, then: selling price = A (1+15%) = B (1+25%).

Yes: the purchase price b = a (1+15%)/(1+25%) = 92%, that is, the purchase price has decreased 1-92%=8%.

3.( 1) Sales decrease =(x-60) 10 pieces.

(2) Let the sales unit price be X yuan, and the sales volume = 500-(x-60)10 =100-10x.

Sales amount = 9000 = (1100-10x) (x-50), ranking: x? - 160x+6400=0

X=80 yuan, and the sales unit price should be set to 80 yuan.