Rates from measurements
Some useful numbers are not a single measurement but one measurement divided by another. What do they tell you that neither measurement can?
Where you are 2 inside
What this area is about
Divide one measurement by another and you get a rate: how much of the first comes with each one unit of the second. A rate compares things fairly when the raw amounts differ in size. Each rate here is a unit rate with measured quantities on both sides, so the same per-one reasoning runs all of them.
How these fit together
Both children are definitions with the problems they answer.
Flow rate is volume per unit of time, such as liters per minute. Knowing it lets you say how long a bath or a pool takes to fill, or how much arrives in a given time.
Density is mass per unit of volume, such as grams per cubic centimeter. It compares materials, not objects: a log has more mass than a pebble, but the pebble packs more mass into each cubic centimeter.
A path through it
The two lessons mention each other, but neither depends on the other. Flow rate needs liters and milliliters and units of time. Density also needs mass units and the volume of a box, so if you are taking both, flow rate is the shorter step in.
Finish here and any “per” quantity built from measurements reads the same way: a unit rate, with units on top and bottom that tell you whether to multiply or divide.