The ideal gas equation is pV = nRT
From that you can derive several equations, depending on which variables are fixed.
1) When n and T are fixed:
pV = nRT = constant
pV = constant => p1 V1 = p2 V2 => p1 / V2 = p2 / V1 ---> Boyle's Law
2) When n and V are constant:
p / T = nR/V = constant
p / T = constant => p1 / T1 = p2 / T2 ----> Gay - Lussac's Law
3) when n and p are constant
V / T = nR/p = constant
V / T = constant => V1 / T1 = V2 / T2 ---> Charles' Law
4) When only n is constant
pV / T = nR = constant
pV / T = constant => p1 V1 / T1 = p2 V2 / T2 ----> Combined gas law.
There you have the four equations that agree with the ideal gas law.
The first one is substance 3
The second one is Oxygen, Helium, and carbon dioxide
The third one is the oil floats on top of the water
Density(D) is defined as Mass(M) divided by Volume(V).
The formula for Density is:
D = M / V.
Another way to remember the formula for Density is to remember "Mass per unit of volume".
I hope this helps!