If the time at GMT is 10:00 a. m. what will be the time at 15 degree west longitude

The meridian 15° west of Greenwich is a line of longitude that extends from the North Pole across the Arctic Ocean, Greenland, Iceland, the Atlantic Ocean, Africa, the Southern Ocean, and Antarctica to the South Pole.

Technically -1 hourso 9:00ambut all countries on that line are running on GMTso it’s same 10:00am there