Question 250380
You need to decrease the water from 50/51 to 20/21 so the Lowest Common Denominator is 21 x 51 = 1071.
The reduction is then from 1050/1071 to 1020/1071 or 30/1071  for 100 gallons of liquid this is 3000/1071 or 2.801120448 gallons roughly.