Question

The countries like USA and Russia have adopted more than one standard time why?

Answer 1

both USA and Russia are huge countries in size and if they have only one standard time then the difference between the real time and standard time of two ends would be very large so to avoid this, these countries have two standard times.

Answer 2

The countries like USA and Russia have adopted more than one standard time because
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these countries are too large and the difference in latitude between the corners of these countries will also be large which wont be proper.

The time should be almost same in all the places in order to select a standard time , therefore to keep the time accurate at its cities or places they have  adopted more than one standard time