The difference between「N分で」and「後N分で」

Question

I know that both of them mean "In 10 minutes" but I don't understand the difference between them.

I have some examples which kind of tell me which one has to be used in different situations but I saw an example when both options can be used.

Here are the examples:

1. 本を1日で読みました。

2. 本は後10ページで終わります。

I can assume that 「後」can/must be used when talking about the end or the beginning of something.

Sentences where both options were used:

1. 車で15分で着く。

2. 車で後15分で着く。

So, what is the difference between with and without 「後」?

Answer 1

I'd understand the two phrases as "it takes N minutes" (in general) and "it takes N more minutes" (in addition to time already spent). Especially the last example reads to me as:

1. 車で15分で着く。 It takes 15 minutes by car. (In general, no mention of whether people are even planning on going)
2. 車で後15分で着く。 Will arrive by car in 15 more minutes. (We're already on our way)

In practice, the first sentence could also mean the same as the second sentence depending on context, but if the 後 is there it would definitely mean people are already on the move.

Answer 2

What you are asking about is equivalent to the difference between "in # minutes" and "after # minutes" in English.

Your first example:

本を1日で読みました。
I finished the book in one day.

You are giving the amount of time it takes, without the use of the word after.

Your second example:

本は後10ページで終わります。
The book will finish after 10 pages.

Here you employ the use of "after" in your given period.

The same applies to your example with the car. The first sentence you say "It takes (me) 15 minutes by car," and in the second sentence you say "(I) got there after 15 minutes of driving."

So the difference made by putting 後 into your sentence is that you are specifically indicating that the verb happens after your period of time.