Question

when current gain and emittor current are given how to calculate base current

Answer 1

This is related to bipolar junction transistor in the common emitter  configuration.  The current gain is defined as α, which is usually around 50. 

A small DC current in  the base of the transistor triggers a high current in the collector region which flows towards the emitter.  The base is less dop ed and collector is heavily dop ed with impurities.

Current gain = collector current / base current
                   =  [ emitter current - base current ] / base current
                   = emitter current / base current - 1

base current =  emitter current / [ current gain + 1 ]