Q2.
SELECT Sum(INV_QTY) AS TTL_QTY
FROM Invhdr
INNER JOIN invdtl ON Invhdr.INV_NO = invdtl.INV_NO
WHERE (INV_ITEM=10101)
AND (Month([INV_DATE])=2)
AND (Invhdr.INV_CUST<>21225);

I used Access to knock this up, its a great SQL calculator.