Transmitter Power Calculations

[damisco2000]

A transmitter normally uses an absolute power level of 100mW. Through the course of needed changes, its power level is reduced to 40mW. What is the power level change in dB?

My answer -10 based on the law of 10 but these are the answers 

The answers were (a)2.5dB (b)4dB   (c) -4dB  (d) -40db

Can someone explain my mistake and how they got -4dB

Also, how is this different from this question below

If the transmit power level of an AP is decreased from 100 mW to 10 mW, what is the power reduction, measured in dB?

my answer was -10dB and i got the answer with this explanation 

The Law of 10s states that a reduction of 1/10 equates to -10 dB.

[patoberli]

Have a look here: https://www.wifi-professionals.com/2018/10/dbm-mw-conversion-cheat-sheet

[Rasika Nayanajith · Accepted solution]

Rememver 1mW = 0dBm which is the reference value. dBm value is equal to 10log(power in mW/reference value).

This will result

0.1mW= -10dBm

1mW = 0 dBm (Reference value)

10mW = 10dBm

100mW = 20dBm

1000mW = 30dBm

To estimate other values we use rule of 10 (10 times in mW = + 10dB or 1/10th of power = -10dB) and rule of 3 (double the power in mW = +3dB or half the power = -3dB).

if you go by that then 40mW = 10mW x 2 x 2 which is same as 10dBm+ 3dB + 3dB = 16dBm

So 100mW-> 40 mW is same as 20dBm -> 16dBm which is -4dBm

HTH

Rasika

*** Pls rate all useful responses ***

[damisco2000]

Thank you so much Rasika, you got it, it is very clear, thanks.