Solution:
If it is 8 o'clock, the clock will chime 7 times after the first chime is heard.
Since the time gap between any two chimes is 3 seconds, this will take 7 × 3 = 21 seconds.

Now, to know what time it really is, you have to wait an 'extra chime gap' to see if the clock chimes again.
Adding the extra 3 seconds, the total time required is 21 + 3 = 24 seconds.

Food for thought:

Does the solution assume that the time taken for the sound of the chime itself is negligible?

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