(1)1个苹果+1个梨=12
(2)1个梨+1个菠萝=20
(3)1个菠萝+1苹果=18
((1)+(2)-(3))/2=7=梨
((1)+(3)-(2))/2=5=苹果
((2)+(3)-(1))/2=13=菠萝