You manage a coffee shop. you begin the day with 25 pounds of coffee beans. you use 4 pounds of beans to brew coffee and sell 20 equal-weight bags of beans. at the end of the day you have 6 pounds of beans left. what is the weight of each bag of beans you sell?

So we started with 25, then after we lost a solid 4 as a given variable. So now we have 21 beans. Later we find that 6 pounds are left and are in not part of the beans sold that can be effectively used to find the weight of each equal bean bag. So we subtract 6 from 21. We get 15 pounds that were sold in 20 bags. The equation to find the weight of each bag from there is : 

15 lb/10 bags 

From there it equals 0.75 lb per bag.

500