When it's about a date of a year, how do I say 2105? Twenty-first 0 five, or twenty-one 0 five?
Thanks in advance :)

This all really depends on how you want to say it. I would personally say the year as "twenty one thousand and five" but I just say years weirdly. I don't know how this answer can help but I hope it does anyways.

[Hey if this is wrong, don't post a rude comment! Message me and I will try to correct the answer!]

the following key ideas correctly express the key sentence.

restate key idea: most people can learn to swim in ten short lessons.

restate key idea: the majority of water-accident victims require mouth-to-mouth breathing.

restate key idea: an empty, tightly closed gallon jug will support a tired swimmer.  

restate key idea: first aid is to prevent accidents as well as assist in rescue.

explanation: the key idea needs to have the same topic as the key sentence (drowning and safety).