<span>You only need an estimate? Great. Think of the necessary material as 6 yards, so ten costumes would take 60 yards. Then think of the fabric on hand as 240 yards, so taking 60 yards away from that would leave [an estimated] 180 yards of fabric.
For an equation, use:
F" = F' - 5.8 C
Where F' is the initial fabric stock, F" is the remaining fabric, and C is the count of costumes.