When there is noon at I.S.T meridian people at another place on the earth are taking their 6 O' clock morning tea. The longitude of the place is:

Answer: (b) 7° 30' W

6 hours behind 82°30′E is 90° to the west: 82°30′E − 90° = 7°30′W.

The idea

For local solar time, 15° of longitude equals one hour. Under Earth’s normal rotation, a place west of the reference meridian has an earlier time. Convert the time gap into an angle before adding or subtracting longitudes.

Find the clock difference → Earlier time means west → Multiply hours by 15° → Subtract this angle from the reference longitude → If the result is negative, write it as west longitude

Why the others are wrong

(c) East would be ahead in time, not behind.

(d) 90°W would be 11.5 hours behind.

The trap

Behind in time = west.

Elimination technique

The target clock is behind IST, so a longitude east of 82.5°E cannot work. Check the remaining options using the time gap, rather than choosing a familiar meridian.

Also know

One degree of longitude corresponds to four minutes of local solar time, and 1′ corresponds to four seconds. The same longitude difference does not represent the same ground distance at every latitude: meridians approach each other towards the poles. Official time zones may depart from local solar time.