甲、乙、丙三位鱼夫出海打鱼，他们随船带了21只箩筐。当晚返航时，他们发现有七筐装满了鱼，还有七筐装了半筐鱼，另外七筐则是空的，由于他们没有秤，只好通过目测认为七个满筐鱼的重量是相等的，7个半筐鱼的重量是相等的。在不将鱼倒出来的前提下，怎样将鱼和筐平分为三份？

-A, B, C 3 fisherman fishing, they brought on board 21 baskets. That night back, they found seven baskets full of fish, with seven baskets in a half-loaded fish baskets, and seven basket is empty, because they did not weigh, that only through a visual seven baskets full of fish is equivalent to the weight of seven semi-fish basket weight of the same. The fish will not reversing the premise, how to fish and divided into three baskets?