Write questions for these answers with How long and the present perfect. 1. They've been in class for 15 minutes. .............

1. They\'ve been in class for 15 minutes. For how long have they been in class? 2. He\'s known John since he was at primary school. How long has he known John? 3. Mary has worked in Miami since for one year. How long has Mary worked in Miami? 5. My mother has had a motorbike since last year. How long has my mother had a motorbike?6. They\'ve studied Chinese for two years. How long have they studied Chinese?