Answer:
a quantitative observation implies that the subject can be measured by quantity, aka amount or in numbers.
Ex 1: adding one gram of salt to one gram of sugar makes two grams of seasoning. in this example, there are individual quantities (1 gram of each) and total quantity (2 grams). this only changes if the substances have a chemical reaction, such as one of them destroying the other, then the weight would change.
Ex 2: a more simple example is the weight of something. putting the substance on a scale (one specifically for whatever you are measuring, whether it be liquid or solid) is the best way to determine its quantity.