The reason is that the average price is 8/16=0.5. price of B is 0.5 already, so we just keep it to be even number. Otherwise, the 0.4m+0.6m cannot be ended up with 0.5.
If you want to make the average price to be 0.5. You have to make A and C average to 0.5. That is, you have to prepare equal amount of both.
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