If I have 2.4 amps on 5 Volts, how many amps am I using on 220v?
$$ P_{OUT} = P_{IN} \cdot Eff $$

Where P is power (W) and Eff is efficeincy.

Since \$ P = VI \$ this can be rewritten as 

$$ V_{OUT}I_{OUT} = V_{IN}I_{IN} \cdot Eff $$

A decent PSU will have an efficiency of about 85% or so.

You should be able to work it out from here.