### Video Transcript

Density equals mass over
volume. Work out the mass when the density
is three grams per centimetre cubed and the volume is four centimetre cubed. Circle your answer. The options are 0.75 grams, 12
grams, one and a third grams and 24 grams.

Well, the first thing we want to do
is take a look at the question. And the question says that we want
to work out the mass. But we’ve got a formula that tells
us the density, so we’re gonna need to rearrange our formula to help us find out
what the mass is going to be.

So, if we have density equals mass
over volume, and we want mass to be the subject of formula, then what we need to do
is remove volume from the denominator. And to do that, we multiply each
side of our formula by volume. So therefore, when we do that, we
get density multiplied by volume is equal to the mass. So, we now have a formula for the
mass.

We could also work it out using the
mass-density-volume triangle. And with this, what we do is you
cover up the one that you’re looking for and you see what’s left. So, if we’re trying to find out the
formula for mass, we cover mass up. And we’re left with D and V. And because D and V are next to
each other, this means multiplied by. So therefore, we could say that
mass is equal to density multiplied by volume.

Okay, great, now, let’s use this to
solve the problem. Well, we know that the density is
equal to three grams per centimetres cubed. And we know that the volume is
equal to four centimetres cubed. So, now, what we need to do is
multiply these together. So therefore, the mass is gonna be
equal to three multiplied by four, which gives us a final answer of 12. So therefore, we can say that the
second answer must be the correct answer. And that the mass, when the density
is three grams per centimetre cubed and the volume is four centimetres cubed, is 12
grams.