Density is an intensive (does not depend on amount) property of the object.
Density = mass/volume, typically units are g/ml
The old standby "Cross Multiply and Divide". This can be used when you have two ratios equal to each other.
Step 1- Cross multiply numerator on the left side x denominator on the right. In our example (ad)
Cross multiply denominator on the left side x numerator on the right. In our example (bc)
So we get ad=bc
Let's solve for the four variables.
Step 2 - Divide,
Solving for a ad/d= bc/d ------> a= bc/d
Solve for d ad/a= bc/a -------> d= bc/a
Solve for b ad/c = bc/c -------> ad/c = b
Solve for c ad/b = bc/b --------> ad/b = c