A transistor is used in an amplifier circuit in common emitter mode. If the base current changes by $100 \mu \mathrm {~A}$, it brings a change of 10 mA in collector current. If the load resistance is $2 \mathrm { k } \Omega$ and input resistance is $1 \mathrm { k } \Omega$, the value of power gain is $x \times 10 ^ { 4 }$. The value of $x$ is $\_\_\_\_$ .