Abag of dog food is on sale for 50% off. if c represents the original cost of the bag of dog food, which expression shows the sale price? a c+0.5 b c-0.5 0.5c d 0.5-c

I am pretty sure it is C.
since the dog food is 50 percent off, you need to divide the price by two. The question doesn't give you the price so you have to multiply the original price (c) by 1/2 (0.5). 
Let's say the price of the dog food is $10. When you think about it without doing the math $10 at 50% off is $5. so you have to multiply the original price by 0.5 (10x0.5=5)
I'm not very good at explaining... I really hope it helped though