 Now sometimes the concept of enthalpy is a little difficult to grasp, so let me give you an analogy. Say you're a stall holder at a market and you're selling cherries. Obviously the amount of money that someone pays for your cherries depends on how much they want. So usually you'll set a unit price of so many dollars per kilo. Now there's obviously a relationship between the price per unit, the price paid by the customer, and the mass of the fruit that they want, and we can write that relationship as the price per unit, which is in dollars per kilo, equals the price in dollars over the mass of fruit that they buy in kilos. So say a customer hears about your cherries and knows that a friend of theirs bought 1.5 kilos of cherries for $12. They could work out the price per unit using this relationship. We've got the price per unit is the price over the mass, and that was the price was $12, and the mass of cherries that they bought was 1.5, and 12 divided by 1.5 is $8 per kilo. So that's the unit price that you're selling your cherries for. Then there's the case where you as the stall holder can use that relationship to work out how much a customer must pay. Say someone wants 2.4 kilos of cherries, so you have to work out the price. So you rearrange that relationship and the price that they need to pay equals the unit price times the mass of fruit that they're going to buy. So the unit price is $8 per kilo times by 2.4 kilos, which equals $19.20. So that would be the price that they paid. So that's the second case where you'd use this relationship. The last case where you'd use this relationship is if a customer comes up and says, I've got $6, what mass of cherries can I buy? Or give me $6 worth of cherries. So you have to work out the mass of fruit that they need. So you rearrange the relationship and you'll get that the mass of fruit equals the price that they're going to pay over the unit price. So they've got $6 over the unit price of $8 per kilo. 6 divided by 8 is 0.75 kilos. So they get that many cherries. Okay, so as long as you know two of these three things, the price per unit, the price of the fruit, or the mass of the fruit, you can work out the third missing thing.