Ответ:
15) Mary is going to stay at the hotel near the sea
16) Are you going to write a letter to Bob?
17) I am not going to go to Brazil next summer. I haven't got any money
18) Kathy and Beth aren't going to make hot dogs. They don't eat meat.
19) My brother is going to do lots of water sports. He loves the sea!
20) We aren't going to swim in the pool. It's too cold.