Can someone explain this A2 transformer question please?

Here's the context:

"When in standby mode, the transformer supplied an output current of 300mA at 9.0V to the internal circuits of the TV set."

Here's part 1:

"Calculate the power wasted in the internal circuits when the TV set is left in standby mode."

0.3*9 = 2.7W (I get this part)

And here is part 2, which I don't understand:

"If the efficiency of the transformer is 0.90, show that the current supplied by the 230 V mains supply under these conditions is 13mA."

I thought you would multiply 2.7W by 9 to get 90% (as 10% of power is wasted due the the transformer being 0.90 efficient) then divide this value by the 230V.
This isn't the case and instead you treat the 2.7W as the power that isn't wasted. So you're supposed to divide the 2.7W by 0.9 to get the total power and divide that by 230V.

If the power on the 230 V side is $P_\text{in}$ and that on the 9 V side is $P_\text{out}$ then we have:

$2.7 = P_\text{out} = 0.9 P_\text{in}$

so rearrange that to get the result in mark scheme.