倍投是什么定律

简介：Ten minutes later, a couple finally came over hand in hand and asked about the price of the watermelon. Su Can showed half of his white teeth and said brightly: This green-skinned watermelon costs 15 cents per pound. Then he pointed to the thick-skinned bearded watermelon and said: This kind of watermelon is more expensive, costing 18 cents per pound.