We have a busy week. From Monday to F ____   , my parents and I are busy. We get up at six in the morning. We often have b  ____    at six thirty. I often have some bread and milk. Then my parents go to work by car and I go to school on f ____   . Our morning classes b ____    at eight. We have Chinese, Maths and E  ____   every day. We also have Science, PE, Art and Music lessons. At school, I have l  ____   of friends. We often talk to each other after class.

   I come b  ____    home from school at five in the afternoon. My parents often get home at five thirty. After dinner, they usually w  ____    clothes and I do homework. I don’t go to school on Saturday or Sunday. O  ____    Saturday evening, I like to w  ____   emails to my friends. My parents often watch TV.

求：

在如图所示的电路中，电源电压U保持不变，定值电阻R=20Ω．闭合开关s，当滑动变阻器R′的滑片P在中点c时，电流表示数为0.4A，当移动滑片P至最右端时，电流表示数为0.3A．则电源电压U与滑动变阻器R′的最大阻值为（　　）