Posted by Anonymous on .
You are buying gifts for 10 people. You decide to buy each person either a CD or a DVD. A CD costs $12 and a DVD costs $20.
Write and simplify an expression in terms of c for the total cost of all the gifts you buy.
My answer My expression is 12c + 20(10c). When I use the distributive property, I get 12c + 200  20c. Now, when I simplify this even further, I get 32c + 200, which is not making sense.

Math 
Steve,
you should subtract that 20c, making
2008c
note if c=0 all gifts are dvd's, costing 200.
Then since cd's are $8 cheaper, you save $8 for each cd bought. 
Math 
Anonymous,
Thank you. Also, the problem asks about what properties of real numbers I used to solve this problem. So far, I am only seeing the distributive property. Is this the only one used?

Math 
Steve,
I suppose associative is also involved, since we have to use it to rearrange the
12c+20020c
to be
200+12c20c
then distributive again to get
200+(1220)c
do you have some property that equates adding a negative and subtracting a positive? 
Math 
Anonymous,
No.