A stereo draws 2.4A at 120V. The audio output power is 50W. a)how much power is

Question

A stereo draws 2.4A at 120V. The audio output power is 50W.
a)how much power is lost in the form of heat in the system?
b)what is the efficiency of the system

Here is what I did:
P=VI
=(120)(2.4)
=288 watts..this is what I used for my input power.

then I used P.input=P.output + P.lost
this gives me 288W=50W + P.lost
solving this I get the power lost is 238 watts.
- this is what doesnt seem right to me because then when I solve the efficiency equation: n% = (P.output/ P.lost) which is 50/238 and the efficiency comes out to 21% which seems pretty low. any help would be appreciated. thanks!

Explanation / Answer

Your math is right for the power p= VI so P = 288 The formula for n is [output / input ] * 100 = n so = 50/288 = 17.4 % this is low but for electronics normal, a lot of energy would also be converted into reading of the music, producing the desired rythm, powering the disply, ect.

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