WALT reason with linear proportions.
This is how I worked it out:
First I drew up the table in my book, I then tried ton work out which option would be the cheapest. I read that if Sally changed from Cellcom to a different provider she would have to buy a new phone. So I calculated the prices and found that Redfone was the cheapest. To get 7GB she would have to get two 512MB for $25.00 in total, then a further 4GB and 2GB (price N/A) to equal 7GB.
Even though the price in unavailable I assume that it is cheap seeing as though the 512MB that she got was the cheapest out of all of them.
My next step is to get a new maths goal.