First of all, distinguish the concept of telling time, because half an hour clock has to ring. If he doesn't look at his watch but just listens, then half an hour is counted as telling the time.

In this case, there are:

10 knock 10, 10 knock 1, * * *1half an hour;

Knock at 9: 30, 10, which is also half an hour. Same as above.

If it takes an hour. Then knock for half an hour, and knock on the hour 10. We know that the sum of two adjacent integers must be odd, so it is impossible.

If it takes an hour and a half, then two and a half beats, leaving nine beats, is exactly equal to 4+5.

So it can be:

Four o'clock, four thirty, five o'clock and five thirty.

If it takes two hours, it can be three whole hours plus two and a half minutes;

That's two, half past two, three, half past three and four.

Or three and a half plus two hours. 1 1-3=8, and 8 is an even number, which is wrong.

If it takes two and a half hours, it is three hours plus three and a half hours, and the sum of three hours is eight, which is impossible.

If it takes three hours, it is three hours plus four and a half hours, or four hours plus three and a half hours.

The former: 1 1-4=7, not a multiple of 3.

The latter: 1 1-3 = 8, 1+2+3+4 = 10 > This situation is unacceptable.

So there are three situations as follows:

Half an hour, an hour and a half, two hours.

If half an hour doesn't count as an hour,

Then only two hours is possible.