Discussion
The opening time of the Central Government offices in India is 9:00 a.m. IST. What will be the local time of opening of the Central Government offices located at 77°30′ east longitude near Bengaluru and 91°15′ east longitude near Agartala, respectively?
Answer: B
The local times are 8:40 a.m. and 9:35 a.m.
IST is the local time of the standard meridian, 82°30′ E. Local time changes by 4 minutes for every degree of longitude: it is earlier to the west and later to the east.
Bengaluru lies west of the standard meridian:
\[ \begin{aligned} & 82^{\circ}30' - 77^{\circ}30' = 5^{\circ} \\ & 5 \times 4 = 20 \text{ min behind IST} \\ & 9{:}00 - 0{:}20 = 8{:}40 \text{ a.m.} \end{aligned} \]
Agartala lies east of it:
\[ \begin{aligned} & 91^{\circ}15' - 82^{\circ}30' = 8^{\circ}45' \\ & 8.75 \times 4 = 35 \text{ min ahead of IST} \\ & 9{:}00 + 0{:}35 = 9{:}35 \text{ a.m.} \end{aligned} \]
The first option gives the same two times in reverse order, which is what you get by adding for a place to the west. The 20- and 25-minute options come from miscounting the 8°45′ difference or mixing up which city is east.
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