Compute the total amount pumped out in the last three days:

210 gallons/day x 3 days = 630 gallons

Add the volume pumped out (removed) to the volume today:

Volume today = 6600 gallons

Volume pumped out = 630 gallons

Volume 3 days ago = 6600 gallons + 630 gallons = 7230 gallons

**Thus the volume three days ago was 7230 gallons.**