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 Question from Ann, a parent: You have 3 fish weighing a total of 21 lbs. The largest fish weighs twice as much as the second fish and the third fish weighs half the amount of the second fish. So how much does the third fish weigh? My answer is 3 lbs., however I don't know how to prove my answer. Can you help. Thank You

Hi Ann,

If the third fish weighs 3 lbs. then that is half the weight of the second fish so the second fish weighs 6 lbs. The largest fish weighs twice as much as the second fish so it weighs 12 lbs. Hence the total weight is

3 + 6 + 12 = 21 lbs.

I don't however think that this is what the teacher wants. The teacher wants to know how you arrived at the answer of 3 lbs. I expect that the teacher is looking for something like this.

Suppose the third fish weighs $x$ lbs. That is half the weight of the second fish so the second fish weighs $2x$ lbs. ....

Continue by expressing the weight of the first fish in terms of $x.$ Add the three weights and set the result equal to 21 lbs. Solve this equation for $x.$

Penny

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