请给出一元二次方程x²－4x＋____ ＝0的一个常数项，使这个方程有两个不相等的实数根

一个水池装有一个放水管和一个排水管，单开放水管8小时可将空池灌满，单开排水管10小时可将满池水排空。如果一开始是空池，打开放水管5小时后又打开排水管，再过多长时间池内刚好灌满水？

根据短文理解，选择正确答案填空。 

    Jim is a junior high school student. He gets up 16:30 every morning. His home is near his school. He 2home after breakfast. He goes to school on foot. This is good for his 3. He has four lessons in the morning. He has lunch at home. After lunch he goes to school again. In the afternoon, Jim has lessons . He likes English and PE lessons. He has PE lessons on Tuesday and Friday. He is good at4football and basketball. He is also good at the high jump. His PE teacher likes5 very much. He often plays football with his classmates after school.