Answer
Greenwich Mean Time (GMT) is the time in Greenwich, England which is at a longitude of $0^{\circ}$.
If our time zone is ahead of GMT, then we are east of the prime meridian. If our time zone is $x$ hours ahead of GMT, that is $x$ time zones east of Greenwich, then our longitude is approximately $(15~x)^{\circ}~E$.
If our time zone is behind GMT, then we are west of the prime meridian. If our time zone is $x$ hours behind GMT, that is $x$ time zones west of Greenwich, then our longitude is approximately $(15~x)^{\circ}~W$.
Work Step by Step
Greenwich Mean Time (GMT) is the time in Greenwich, England which is at a longitude of $0^{\circ}$.
If our time zone is ahead of GMT, then we are east of the prime meridian. If our time zone is $x$ hours ahead of GMT, that is $x$ time zones east of Greenwich, then our longitude is approximately $(15~x)^{\circ}~E$.
For example, New Delhi, India is 5 time zones ahead of GMT. New Delhi's longitude is approximately $(5)(15^{\circ})~E$ which is $75^{\circ}~E$.
If our time zone is behind GMT, then we are west of the prime meridian. If our time zone is $x$ hours behind GMT, that is $x$ time zones west of Greenwich, then our longitude is approximately $(15~x)^{\circ}~W$.
For example, Los Angeles, California is 8 time zones behind GMT. Los Angeles' longitude is approximately $(8)(15^{\circ})~W$ which is $120^{\circ}~W$.
