If Rosy had x friends with her, the total cost is 1200/(x + 1)
Which is the price per dish?
LEt's say that Rosy has x friends, so the people that must pay the bill is x + 1
(the x friends plus Rosy)
Now we need to divide evenly the total cost between the number of people, that will be the cost per dish, we will get:
Cost = 1200/(x + 1)