Which is more dense—a stack of 5 NT coins or a stack of 20 NT coins?

A stack of 20 NT is denser than a stack of 5 NT, because the volume and mass of a stack of 20 NT is bigger than the volume and mass of a stack of 5 NT. To calculate the density of a matter, just use mass/volume, so maybe heavier or bigger stuff is denser.

Why do we study MASS and VOLUME for 3 days?

The reason that we are learning mass and volume for these three days is because we were learning about matter. I guess you would ask what does matter related to mass and volume? I'll say matter is made up by mass and volume, matter is stuff that takes up weight(mass), and space(volume). Basically what are the two important things for studying matter is what is matter and can matter disappear? I think matter can disappear, and when matter disappear, it means the weight and the space that it takes up will be gone too.

Is air matter?

( source:http://i.treehugger.com/images/2007/10/24/air-quality-clouds.jpg)

When I first think of matter, I think matter is the things that have weight and takes space. The reason that I think air is not a matter it's because air is invisible, it seems that air don't takes space and it doesn't have weight either. After reading some articles about air and matter, I finally understand it and I found out the 3 big categories of matter: gas liquid, solid. Air does take up space, even though we can't see it, and air has weight, even if we can't feel it. I am not lying, this had been observed and analyzed by some scientists.