If I understood the requirement correctly, this can be done in several ways, for example something like:
SELECT a.Billing_Acct,
a.Multiplier,
a.Total,
c.multiplier * c.total as result
FROM TableName a,
(SELECT b.Billing_Acct,
AVG(b.Multiplier) AS multiplier,
SUM(b.Total) AS Total
FROM TableName b
GROUP BY b.Billing_Acct) c
WHERE a.Billing_Acct = c.Billing_Acct;