Copper Bus Bar

Your teacher is trying to teach you about density, you may want to look it up.

Weigh it on scales.

Or work from 8.96gm-cm3

The answer is 42. I'll leave it to the student to find the reason.

For that he'd need Deep Thought

Apart from the jokes above (which I like), the easiest way for a simple approximate check is as follows :-

width * height * 8.9 / 1000 = weight in kg per metre length

so for your example 25 x 3 = 75, x 8.9/1000 = 0.6675 x 20 = 13.35kg

Hope this helps you and others.

2 days to get his homework done for him.

Could do better.

What's the point in a school or collage setting homework if we do it for them!

Maybe the point is that if we keep doing his work then our jobs will be secure. Obviously by not doing this simple work he will never be able to do the real work.