Question

Calculate the Time of Place B 60°W  when it is 9 pm at prime meridian.

Answer 1

Alright, it is 9 pm at prime meridian, which is at 0 longitude. When we move toward West, it should be earlier (because the planet rotates East to West, counter-clockwise if we look at it above the Norther hemisphere).

360 degrees (the entire circle of the planet) are divided by 24 hours, or each 15 degrees contain 1 hour (360/24=15°/h).

60° contain 4 times of 15°/h, therefore, going 60° West, the time is

$$9\text{PM}-4\text{ hours}=5\text{PM}.$$