Here’s a real life problem I gave my students.  It was a lot easier to explain, than to write here as a problem. :).  My students were highly motivated, as I was going to make my decision that morning, based on their findings.   It was great having the students share their solutions with the class, as there were many different methods of solving:


Yesterday I had someone paint the spare room in my house.  When I came home I was dismayed that the color I chose was much darker than I had anticipated.  I went back to the paint store to try again.  Since I liked the color I asked for a lighter version.  The clerk made a “batch” which had a color strength of 25%.  It was too light.  He informed me that he was only able to mix in increments of 25%, so he made a batch of 50%.  It seemed a bit dark.  He suggested I take this 50% strength gallon home along with a quart of white paint, and a large pail for mixing.  I painted a swatch of the 50% on the wall and checked it in the morning.  I was still undecided.  Since my painter was coming back that morning, I needed to make a decision as to whether to mix the gallon of 50% and the quart of white.  The question posed to my students was:  What would the color strength (%) of the mixture be, in relationship to the original paint. 


Note:  most of my students came up with the same wrong answer to begin with, but when I prompted them to try again, they were able to determine the correct percent.