Question 386597
c o g(a) is the application of c on g. 
 
here g gives the number of gallons, and c their price, hence (c o g) makes sense.
 
g o c would take the price of n gallons but then those are not an area, and hence cannot be argument of g, hence g o c has no sense. 
 
(c o g)(a) were the price of gallons to cover a floor with area a.