这个我还真查了一下

With multiplication you have a multiplicand (written second) multiplied by a multiplier (written first). The unit for the multiplier has to be sets of the unit for the multiplicand. For example, if you have 3 bags each containing 5 apples, then you can multiply to give

[3 BAGS] x [5 APPLES PER BAG] = 15 APPLES

Note how the units cancel: BAGS X APPLES/BAG = APPLES