Context operator in Webi

Dear experts,

     I have a requirement to create a Webi report, based on the below 2 tables. I'm relatively new to BO. Can some one please help me with this.

I'm looking for the output to look exactly like Final Report in the below.

Latest Amount : The value should be the Amount  field from Invoice_Table whose sequence is minimum for that PO (see I2(300) sequence 1 for PO1, I5(600) sequence 2 for PO2, I6(100) sequence 1 for PO5.).

Total Amount of PO : This field should give the total amount for that PO   (PO1-900(100+300+500), PO2-1900(400+600+900), PO5-300(100+200)).

Thanks for all the support.

Regards

-Amit.