A few things about working with dB and dBm.
• Adding dB values is the same as multiplying with regular numbers. So if you add 10dB to a decibel value it is the same as multiplying a regular number by 10.
• Subtracting dB values is the same as dividing with regular numbers.
• It is OK to add dB values to an initial dBm value. This is the same as starting with an input power level and adding amplification or subtracting attenuation from that power level. The final answer will be your output power level in dBm.
In the figure above we have an input power level of 10dBm to which we add 20dB of amplification. The result is an output power of 30dBm. This is the same as starting with 10mW of input power and multiplying that by a factor of 100, giving an output power of 1,000 mW.