A transistor is operated in common emitter configuration at $V_C= 2V$ such that a change in the base current from 100 μA to 300 μA produces a change in the collector current from 10 mA to 20 mA. The current gain is:

50

Current gain in common emitter transistor is

$β=\frac{ΔI_C}{ΔI_B}=\frac{(20-1)mA}{(300-100)μA}=\frac{10×10^{-3}A}{200×10^{-6}A}=50$