The most common disease in the world is the cold. It often starts with a sore throat. You sneeze and your nose runs. You usually have a headache, too. Often you have a cough later. It's not a serious disease, but you can feel quite bad.

    There isn't a cure (良药) for a cold, but you can take some medicine to make you feel better. For example, you can take aspirin (阿司匹林) to get rid of (去除) your headache. It is good to rest. and to drink a lot of water, too. A doctor once told me. "With the right medicine. a cold will last for seven days. With no medicine, it will go on for a whole week!"

    A long time ago, people understood that some plants made them feel better. For example, the juice of lemons (柠檬) makes a sore throat feel better. In modern times, scientists have found out which chemicals (化学药品) are in them. Man, of our medicines today are made from those chemicals. There are some diseases that we can't cure yet. And nobody has found a cure for old age. But because of modern medicines, the average people now live longer than their grandparents.

如图所示，将长方形ABCD的纸片沿EF折叠，点D、C分别落在点D′、C′处，若∠AED′=50°，则∠EFB的度数为____．