Question

calculate the time of london (0° longitude) when the time at Allahabad (82.5°E longitude) is 12:00 noon​

Answer 1

The longitude is irrelevant. Allahabad is in the IST time zone (UTC+5:30).

so when it’s noon at London, the time can be 12:00 GMT (effectively the same as UTC), or it can be 12:00 BST (11:00 UTC). So when it’s noon at London, it can be 16:30 or 17:30 in Allahabad.