for 30 days such that each member of the crew would get 2 quarts per day. If 250 gallons become contaminated after the 10th day and cannot be used for drinking, how many quarts will each crew member get per day for the rest of the trip?

One can use conversions to quarts

Initial supply = 50 CM * 2 qt/(day * CM) * 30 days = 3000 qts

In 10 days the crew consumed: 50*2*10 = 1000 qts

Loss due to contamination is: 250 gal * 4 qts/gal = 1000 qts

Remaining H2O Supply (qts) = 3000 - ( consummed + loss to contamination) = 3000 - 2000 = 1000 qts

Remaining days = 20

Remaining Daily supply (assuming dispensing the same amount each day) = 1000/20 = 50 qts / day

Assuming each crew member gets the same proportion then 50 qts/ day divided by 50 CM = 1 qt / day