Xiao Wang and Xiao Li come to a grain store twice a month to buy rice at the same time. The price of rice has changed twice. The price of rice is x yuan / kg for the first time and Y yuan / kg for the second time?

The average price of Xiao Wang is (x + y) / 2 {suppose Xiao Wang buys m gram rice every time, the average price = (XM + YM) / (M + m) = (x + y) / 2} 2, the average price of Xiao Li is 2 / (1 / X + 1 / y) {suppose Xiao Wang buys m gram rice every time, the average price is (XM + YM) / (M + m) = (x + y) / 2} 2

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