This expression, 1.5p + 3p + 2.5p, will represent the amount of money received when p people buy one of each food item.
Since the price of each is the coefficient for p, this will represent how much money will be gained from each food item. All 3 terms will be added together to represent the total amount of money gained from selling all 3 items.
If you write .6 and .7 as fractions you get 6/10 and 7/10. If you double those you get 12/20 and 14/20, so one potential fraction between the two could be 13/20.
