Calculating the Efficiency of a Transformer for a Small Appliance

[bleedblue1234]

Homework Statement

The specifications of a transformer for a small appliance read "Input: 110 V, 6.4 W" and "Output: 8.9 V, 270 mA". What is the efficiency of this transformer?

Homework Equations

P = I * V

V = I * R

The Attempt at a Solution

6.4 W / (8.9*(270*.001)) ---> I think this is wrong

I am just not sure how the voltage drop of the transformer would affect the power output...

 

[shungun]

Your reasoning is absolutely correct, voltage drop theoretically shouldn't affect the power output, but in theory we assume that there is no losses, no heat generated etc.
Thus when such a question pops up (power out)*100/(power in ) will give you the efficiency.