Check the picture below.
the function is the Cost function for the bagels, of course that'd depend on the price of each bagel, so the cost C(x) is dependent on how many independent bagels Naomi is buying, as you can see in the table in the picture.
now, if Naomi gets 10 bagels, that simply means that x = 10, thus the price is C(10), or namely C(10) = 1.25(10), so Naomi better check her pockets for $12.50 for the bagels, hopefully she'll get some cream cheese too and cocoa.