John usually goes to school by bus. He leaves (离开)his house at seven and gets to school at about seven twenty. His school is on the hill.

    From Monday to Friday John usually has four classes every day. He likes English. He wants to be an English teacher. He hopes he will go to the USA to study English one day. On Sundays, he often goes to the library with his friends, but sometimes they go to see movies.

一片牧场，每天生长草的速度相同．这片牧场可供14头牛吃30天，或者可供70只羊吃16天．如果4头羊的吃草量相当于1头牛的吃草量．那么17头牛和20只羊一起吃这片牧场上的草，可以吃多少天？