You are standing by the duck pond with two pails. one holds 7 gallons and the other holds 3 gallons. neither one of them has gallon markings on the side. how can you get exactly 5 gallons of water in the big pail?

Tricky, but with a bit of guess and check I got to my answer (I don't know how else to do it).

You'll notice that 7-3=4, so if you filled the large pail up to the top and filled the smaller one up from the larger one, you'd have 4 gallons. Next, since you have 4 gallons, you dump the 3 gallons from the smaller pail and fill it up again. You now have 1 gallon in the larger pail. Put that 1 gallon in the smaller pail after dumping the smaller pail yet again. Now, you have 1 gallon in the smaller pail and 0 in the larger pail. Fill the larger one up again, and you have 7 gallons in the larger one. Fill up the smaller pail, and since it already has 1 gallon, you have 3-1=2 gallons left to fill up. As 7-2=5, we have 5 gallons left in the larger pail!