It's twenty minutes' walk 为什么不用twenty-minute?

It's twenty minutes' walk 为什么不用twenty-minute? a twenty-minute-walk or a twenty-minute's walk twenty minute,twenty minutes,twenty minutes',与twenty-minute的 twenty minute's walk用什么提问 a twenty minutes' walk 与a twenty-minute walk哪种说法正确? It's twenty ________(minute) walk to the hospital,let's be _ It is about twenty minutes‘a walk.对 twenty minutes‘a walk提问用 It's abput twenty （ ）（minute）walk to get there how far is it?it's a twenty minutes' walk from here. It's about twenty minutes ' walk to the park 同义句 It's about twenty minutes' walk from here .保持原意改写句子 It's twenty minutes'walk from my home to the school.改为同义句it_