Question

When it is 2pm in Greenwich ,what is the local time at: (a) Mumbai (73° E) (b) Chicago (87°30' W) (c) Sydney (151° E)

Answer 1

Your formulation “2 pm in greenwich [sic]” is ambiguous. Do you mean 14:00 GMT, or do you mean 14:00 in that location?

If it’s the former, ’ answer is all you need. Otherwise you must consider the possibility that BST (British Summer Time, GMT+1) is in effect. During this time AEST is in effect in Sydney, so the time there will be 23:00.

Note that the longitude of Sydney is completely irrelevant.