13 In problems 7 and 11, Leisureland and Busyland each produce and consume 100 kilograms of food and 100 litres of sunscreen per month, and they do not trade. Now the countries begin to trade with each other. a What good does Leisureland sell to Busyland, and what good does Leisureland buy from Busyland? b If Leisureland and Busyland divide the total output of food and sunscreen equally, what are the gains from trade for each country?