If it is 10:00 a.m. at Greenwich Mean Time (GMT), then what will be the time at 90° E longitude?

The Earth completes a full rotation of 360° in 24 hours, which translates to a rate of 15° per hour or 1° every 4 minutes. Greenwich Mean Time (GMT) is the mean solar time at the Prime Meridian (0° longitude). Since the Earth rotates from west to east, places located to the east of Greenwich are ahead in time, while places to the west are behind. To find the time at 90° E longitude, we calculate the difference: 90° divided by 15° per hour equals 6 hours [1]. Because the location is in the East, this time difference must be added to the GMT. Adding 6 hours to 10:00 a.m. GMT results in 4:00 p.m. (16:00 hours) at 90° E longitude [1].

Sources

1. [1] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 2: The Earth's Crust > Longitude and Time > p. 11