Hi Ann,

I can prove that your answer is correct as follows.

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.

Hence your answer is correct.

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