Conversion of transmitter power from Watts into dB I've been practicing problems relating to the dB conversion and came across this interesting example problem.



In the above example Tx power is defined in Watts and the conversion from the Watts to dB is done. However, so far according to my understanding of the definition of dB, is defined in relative to receiver power too. But in the above example the calculation was done just by considering the Tx power. 

Can some one enlighten me? If I'm understanding it wrong or the example is wrong?

Thanks in advance.
Thanks to @MBaz who pointed out my mistake: dBm and dBW are unitless, but they relate to  a reference of 1 mwatt, and 1 Watt. 

The question has an error, as well as the answer. Watts does not convert to dB, it converts to dBW. (i) should be as follows:

find (i) the transmit power in dBm and dBW. 

The reason behind that is that dB is a ratio, while dBm and dBW are ratios with a reference to 1 mWatt and 1 Watt, respectively, and you can use them to recover an absolute value of the power.

For example, the signal to noise ratio is a measure of how strong a signal is compared to the noise. In the linear scale it is written as:

$\frac{\text{Signal Power (in watts or miliwatts)}}{\text{Noise Power (in watts or miliwatts)}} = SNR (\text{unitless})$

or in logarithmic domain: 

$\text{Signal Power (in dBW or dBm)}-\text{Noise Power (in dBW or dBm)} = SNR (\text{in dB})$

So the correct answer to (i) is 

TX power = $10\log(50)=17\text{ dBW}=17+30=47\text{ dBm}$