I need to make 72 cakes for when my guests visit tomorrow. I make the cakes in batches of 12. It takes me 40 minutes to prepare each batch and then they need to be in the oven for 25 minutes (during which time I can be working on the next batch). Once they have finished baking, I need to spend 5 minutes putting the cakes out to cool before the oven is ready for the next batch. It is now 1pm.