Please help to answer a few questions: interesting math questions, classic math questions and pictures, couplets and jokes related to math, introduction of learning methods? thank

1. There used to be 10 birds in the tree, but then three flew away and seven flew in. How many birds are there in the tree now?

2. A person earned 4 1 yuan on the first day, but spent 27 yuan at home; The next day I earned 57 yuan, and his family spent 30.3 yuan. How much is left?

3. Someone filled a glass of water, drank the glass of 1/3, added the glass of 1/5, drank the glass of 1/2, then added the glass of orange juice of 1/3, and drank 2/3 of this drink. How much is left in this cup now?