Discrete vs Continuous Variables
A discrete variable represents something that can be counted. The number of trees in a backyard, the number of 5's in your license ID, the number of cars in a parking lot, etc, are examples of discrete variables.
The continuous variable represents something that cannot be counted but rather measured. The length of a table, the volume of a pool, the voltage of a power source, the temperature of the water, etc, are variables that cannot be counted. They need to be measured.
The first three variables are continuous:
a) The mass of the solid impurities per milliliter of Los Angeles tap water.
b) The amount of coffee in a cup dispenser
c) The length of an arm of a heavyweight boxer
The last variable can be counted, thus it's discrete:
d) The number of children in a household