Maximum DC offset voltage
Assuming that you have given us the complete problem statement, this is a simplified example where they have ignored input offset current.

You are supposed to calculate the output voltage resulting from an input offset voltage of +/- 10mV and also the output voltage resulting from an input bias current of +10uA flowing into each input. Add the two in the way that maximizes the magnitude of the output voltage (worst case).

There will be an error from the nominal input bias current even in the ideal case because the designer has failed to match the input resistances seen by each input. Sometimes that is not important (eg. CMOS-input op-amps) but in the case of such a heavy input bias current as in this example it is a good idea). 



Since OP has long gone, might as well solve it.. 

The input offset is (worst case) the bias current multiplied by the resistance imbalance at the inputs (900 ohms vs. 800 ohms or 100 ohms difference). That is 1mV. So add that to the 10mV Vos and we get 11mV. 

Output offset is gain times input offset or 55mV, worst-case, same answer as Trevor.