Please some help me with this problem, I have complete mental block at the moment.
what I am trying to achieve it seems very easy but cant get my head around it...

1.User will select Sales Person Name from drop down list
2.User will select Product Sold by Sales Person
3.User will add Quantity manually
4.User will click Add Button...
5.Table should be updated accordingly - Quantity should be added to existing cell value
e.g. 'David' has Already sold 4 'HTC XL' and as per current selection the Total in Cell D6 should change to '5'

Please refer to attached Excel Document.
sample sales tracker.xlsx