After 10 minutes 10×2=20 gallons would be added to the 10 gallons of water that was in the drum, so, if there's no overflow, there will be 10+2×10=30 gallons in all. The concentration of contaminant is now found to be 0.01lb/gal, so in 30 gallons 0.3lb of contaminant would have been initially spilled into the drum.
(If the drum overflows as more water pours into it, all the contaminant would have been flushed out by displacement, because the drum can only contain 10 gallons.)