My teacher gave me an exercise which is adapted from the book "Macmillan Advanced Language Practice with key"

Complete the second sentence so that it has a similar meaning to the first sentence, using the word given. Do not change the word given.

(a-2) By the time Tony ____________.

I understand the sentence a-1 as: First, Tony read the book. Then, he knew the answer.

So, my rewrite in sentence a-2 is:

However, when I checked the book's answer key, it said:

I seem not to agree with the book's solution, because I understand that "by the time" means "earlier than or equal to the time"; as a result, for me, the meaning of the book's solution is that first, Tony knew the answer, then he read the book. And it isn't similar in meaning to the original sentence.

Could you kindly tell me whether I correctly understand the meaning of the book's solution?

• The Past Perfect stands here for a completed/finished action. As for the sequence: "he found out the answer before he finished reading". I guess somewhere in the middle of the book. – SovereignSun Oct 2 '17 at 17:53

While your answer is grammatically correct, it's not logical. When you use the expression "By the time X happened, Y happened" the focus is on Y, not X. This establishes some kind of significant relationship between X and Y, and provides a time frame for when Y occurred.

By the time the police arrived, the criminals had escaped.

In this example, we are more concerned about the criminals getting away than the police arriving. If you reverse the order, it doesn't really make sense:

By the time the criminals escaped, the police had arrived

Here the focus changes to when the police arrived, which seems odd, because there is no significant relationship between the criminals escaping and then the police arriving. One didn't cause or affect the other.

That being said, your confusion is justified, because the exercise is flawed: