Example: if the power out is 20 watts and the power in is 10
watts what is the increase in power expressed in dB?
You should know that a doubling of power in fact is 3dB increase so let's
see what the calculator gives !! 

10
20
10

Using a double image of the calculator screen to bring the full input to
the calculator you can see the figure comes out at about 3dB what we expected 
Example: if the power out is 400 watts and the power in is 0.4 watts what
is the increase in power expressed in dB? 

10400
0.4 
Using a double image of the calculator screen to bring the
full input to the calculator you can see the figure comes out at about 30dB
what we expected as 30dB represents a 1000 times power increase. 
If the question was set using voltages then you should now
be able to understand how to do the calculation. 
What is a measured power of 400W expressed in dBW ? 
Remember dBW = 10 log_{10} (power in watts) 


10400 
Example: What is a measured power of 12dBW expressed in Watts
?
For this was have to introduce a secondary key stroke of a previously used
key and one you may have forgotten.
The shift key (use last when turning off the calculator) and the
key which when used "following" (not together with!!) the shift
gives "antilog". 
So what was the question?
Example: What is a measured power of 12dBW expressed in Watts ? 

Remember Power(Watts) = antilog (dBW/10) 
1210
Result 16dBW when rounded up. 
Example: Your transmitter have a power output of 20dBW
and the feeder has a loss of 3dB, the antenna has a forward gain of 9dB.
What is your ERP in watts (effective radiated power) ?
Now we know that Power(Watts) = antilog dBW/10
so Power(Watts) = antilog ((203+9)/10) = 398W rounded
up = 400W 

203910
Result 398W rounded up = 400W 


The brackets are used as they appear
in the formula OR AS NEEDED TO KEEP THE MATH'S RULE TRUE and thus
it makes keying the calculator straight forward
BUT if you have been taught another way then
use it !! 