Ajar containing 140 marbles weighs 20 ounces, and the same jar containing only 100 marbles weighs 16 ounces. what is the weight of the jar?

You should first calculate the weight of each marble 

20 - 16 = 4 ounces difference between the 2 jars
140 - 100 = 40 marble difference between 2 jars

4 ounces divided by 40

4 ÷ 40 = 0.1 ounces per marble 

So each marble is .1 ounces

0.1 ounces times 100 marbles (in second jar)

0.1 × 100 = 10 ounces of marbles

Jar + 10 ounces = 16 ounces

Jar = 16 - 10

Jar = 6 Ounces