Multiple choice general knowledge Which one of the following longitudes determines the Indian Standard Time? 85.5° E 82.5° E 84.5° E 86.5° E Reveal answer Fill a bubble to check yourself B Correct answer Explanation Indian Standard Time (IST) is calculated from 82.5° E longitude, which passes through Allahabad (Prayagraj). This meridian was chosen as it corresponds to the mean longitude of India, 5.5 hours ahead of GMT.