If a recipe, or food label, tells you that your food should take 20 minutes at a certain temperature, and by that time the food is not cooked, don't serve it anyway! Leave it in for a while longer.
My husband is terrible at this. "It said cook it for 30 minutes on the packet" he growls, "So why isn't it cooked?"
This can actually be for a number of reasons. If you are cooking meat, for example, it may be cut thicker than the piece that was used to test the recipe. Or your oven thermostat may be slightly incorrect, meaning that the oven isn't as hot as it is meant to be.
Hopefully this temperature conversion chart and notes will help you to cook and serve good, tasty food from your oven.