30 pieces: Store A: 30 * x-10 * x/2 = 25x; Store B: 30*X*0.6= 18X. Of course, store B is cheap.
If 50 yuan is 60 yuan, the same B store is cheaper, because A store only offers 10 yuan, but the base is increasing.
Or this is more general.
Suppose the price is x and the number of purchases is y.
A store: X*Y- 10*X/2=(Y-5)x, and B store: xy * 0.6 = 0.6y * x.
Bring in 30, 50, 60, and Y-5 is always greater than 0.6Y, so store B is cost-effective.