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To make a preliminary guess about the standard meridian of a country, take the westernmost longitude and the easternmost longitude of that country, take the mean, and find the nearest multiple of 7.5 degree. However, if the country has a large longitudinal expanse, there may be more than one time zone (and hence multiple standard meridians). Location of standard meridian can also be affected by political considerations, demographics, etc. (Nepal and Chatham Islands are the only two exceptions to the 7.5 degree rule, as they have time zones of UTC +05:45 and UTC +12:45.)
Making the first guess:
Longitudinal extent: 68° 7′ 53″ E to 97° 24′ 47″ E.
Nearest multiple of 7.5°: 82.5°
First guess of standard meridian: 82° 30′ E
This is the actual standard meridian of India.
Time zone: 82.5 × 4 minutes ahead of GMT = 5.5 hours ahead of GMT.
Longitudinal extent: 73°49′ E to 134°45′ E
Mean: 104°17′ E
Nearest multiple of 7.5°: 105°
First guess of standard meridian: 105° E
China is longitudinally large, and should ideally have more than one time zone, however, it has only one. Probably for reasons such as demographics (China has far a lot more population in its eastern half than western half), the standard meridian is further east: 120° E.
Time zone: 120 × 4 = 480 minutes ahead of GMT = 8 hours ahead of GMT.
So, longitudinal extent is only a starting point. Every country has its own choice of standard meridian and we can know it for sure only factually, not by calculation.