Suppose you had a jar of marbles, and you wanted to know the average mass of just one of the marbles. You know the marbles all vary slightly in size and composition, so just one marble is not necessarily representative of all the marbles. However, for your purposes, whatever they may be, an average value would work perfectly. As long as you had a mass scale, this would be relatively easy.
Place the jar of marbles on a mass scale and record the total mass.
Dump the marbles out and count how many of them you have.
Place the empty jar on the mass scale and record its mass.
Subtract the mass of the jar from the total mass. This will be the mass of all the marbles. As an example, if you previously measured a total mass of 2,100 grams and the jar had a mass of 800 grams, then the mass of the marbles would be 1,300 grams.
Divide the mass of all the marbles by the number of marbles that were in the jar. In the example, if you had 100 marbles, you would divide 1,300 by 100, which would give you an average mass per marble of 13 grams.
You could also use this procedure for weight by measuring in pounds. However, unlike grams, pounds is technically a unit of force, and not mass.