Average cost calculation after Return to Vendor

Hi,

How is the new average cost calculated in the two scenarios below:

Scenario 1:OH Qty becomes 0 after the retrun to vendor transaction:

1.Current OH qty = 2 and avg cost = $3

2.Return to supplier is done for qty = 2 and the PO price is $10

Scenario 2:OH Qty does not become 0 after the retrun to vendor transaction:

1.Current OH qty = 2 and avg cost = $3

2.Return to supplier is done for qty = 1 and the PO price is $10

Thanks

Rahul