Question

Why standard meridian should be divisible by 7.5?

Answer 1

hyy ...

⭐it is because 360º divided by 24 hours gives 15º per hour as one time zone & hour zones placed 15ºapart. So 
±7.5º is the norm, around a country's standard meridian.
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hope it helps☺

Answer 2

Because 15 degrees of longitude is equivalent to 1 hour in time (that is 360 / 24) it is convenient to adopt time zones at 7.5 degrees or 15 degrees multiples. In other words time zones are separated by multiples of 1 hour or 1/2 hour.

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