A singular non is the opposite of a plural noun. A plural noun names more than one person, place, thing, or idea. A plural noun includes things like "children," "books," "libraries," or "liberties." A singular noun would be "child," "book," "library," and "liberty." Singular nouns only name one person, place, thing, or idea. Hope this helps.
You would write it in the casual form. You wouldn't write it informal because it would sound too unprofessional. And making it formal, well, some people wouldn't understand if you were to write in the formal way. They wouldn't understand the words. So just the casual way would do.
It is personal zone I had a question on my test just like this Glad to help :)
Cost price
Tax
Profit
Add these together to get the answer