In the second grade, I shouldn't have learned equations or decimals, so I have to solve them in other ways.

Replace all the big cows with calves, and you can drink 100/2=50 barrels.

Then 100-50=50 barrels is the difference in drinking water after calves are replaced with cows, and the difference in drinking water for every two cows is 3*2- 1=5.

So Daniel is 50/5*2=20.

The calf is 100-20=80.