数学题：一个水桶里装满了水，第一次倒出水的一半还多1千克，第二次又倒出余下的水的一半还少1千克，这时桶里还剩下6千克水。原来桶里有多少千克水？（四年级下册基训P65）

解答：画一个示意图在最下面。这题需要运用倒推法，一步一步地还原出答案。

6-1=5（千克）………………余下的水的一半

5+5=10（千克）………………余下的水

10+1=11（千克）………………一桶水的一半

11+11=22（千克）……………………一桶水

答：原来一桶水重22千克。

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