Final answer:
If a star crosses the meridian as seen from Greenwich, England at midnight, it will cross the meridian for someone at a longitude of 90° West at 9 PM.
Step-by-step explanation:
The Earth rotates 360 degrees in 24 hours, so each hour it rotates 360/24 = 15 degrees. Therefore, if a star crosses the meridian as seen from Greenwich, England at midnight, it will cross the meridian for someone at a longitude of 90° West at 9 PM. This is because 90° West is 6 hours behind Greenwich time, and 6 hours before midnight is 6 PM.