CAUTION!!!!!!!!

Be sure to read this only if you have tried the problem yourself.

The benefit of brain builders comes from the work you make your brain do to figure them out.

Looking at answers before doing the problem, destroys the power to make you smarter.

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ANSWER:

1. fill the 3 gallon bucket and then pour it into the 5 gallon bucket
2. fill the 3 gallon bucket again
3. pour water from the 3 gallon bucket into the 5 gallon bucket until it is completely full
4. you now have exactly 1 gallon left in the 3 gallon bucket
5. empty the full 5 gallon bucket
6. now pour the 1 gallon of water into the empty 5 gallon bucket
7. fill the 3 gallon bucket again
8. finally pour those 3 gallons into the 5 gallon bucket which still has 1 gallon in it
9. VIOLA ...... EXACTLY 4 gallons are now in the 5 gallon bucket.