The average power used by a stereo speaker is 55 W.?

Question

Assuming that the speaker can be treated as a 4.0 Ampere resistance, find the peak value of the AC voltage applied to the speaker.

Answer 1

(Vrms^2)/4 = 55W

Vrms = (220)^(1/2)

Vpeak = Vrms * 2^(1/2) = (440)^(1/2)

Answer 2

Although your problem statement was ambiguous since you stated 4 "ampere" resistance and resistance should be OHMS. So it was unclear what you meant.  If you really meant resistance then (see reference):

P = (1/2)I*V where:

P = average power

V = peak volotage

I = peak current

Ohm's law: V = I*R gives I = V/R

P = (1/2)(V/R)V

v^2  = 2P/R

V = SQRT(2P/R) = SQRT(2*55/4) = sqrt(27.5)

V = 5.244 volts