Question about transformers

In the transformer shown in the picture, the load resistance RL= 50 ohm. The turns ratio N1/N2= 2.5, and the rms source voltage Vs = 80 V. If a voltmeter across the load resistance measures an rms voltage of 25 V, what is the source resistance Rs?

picture:
http://imageshack.us/photo/my-images/9/img1078yi.jpg/

formulas:
V1/N1 = V2/N2
V2 = (N2/N1) * V1
I1V1 = I2V2

I have no clue how to even begin or

The load voltage is 25V, so:

V1/N1 = V2/N2

V1 = V2*N1/N2 = 25*2.5 = 62.5 V

The source voltage is 80 V, so the voltage drop is:

Vd = 80 - 62.5 = 17.5 V

The current in the load is:

I2 = V2/RL = 25 / 50 = .5A

V1*I1 = V2*I2

I1 = V2*I2/V1 = 25*.5 / 62.5 = .20 A

The source resistance is:

Rs = Vd / I1 = 17.5 / .2 = 87.5 ohms