That is basic transformer theory.
The transformer has a ratio of 100:1 so the secondary voltage (on Rb) is 1/100 of the primary voltage. The secondary current is 100 times the primary current.
Ignoring Ra for now, if the primary voltage is Vp, the secondary voltage is Vp/100. That means the secondary current is Vp/100/Rb.
The primary current is Vp/100/Rb/100 = Vp/(Rb * 10000)
which is the same current as you would get if there was no transformer and the load resistance were 10000 * Rb
Then add Ra back in.