Question 732124
One of the basic ideas is, the company will purchase whole numbers of any sizes of cans and must use the highest quantity of the 5 gallon size cans with remainder being purchased as the smaller 1 gallon size cans.  Note from the description that the 5 gallon cans are less expensive than the 1 gallon cans.   


Let's say, the unit of money is "cash".  The 1-gal cans are 1 cash per gallon and the 5-gallon size is 4 cash per 5 gallons.  


How many gallons of paint needed?
Radius is 2.5 feet.  
{{{(pi*2.5^2*10)*20}}} feet^2 of surface to paint.


Using now the ratio of 1 gallon covering 350 ft^2, 
{{{(pi*2.5^2*10)*20*(1/35)}}} gallons of paint needed.


NOW you can figure what is the largest number of 5-gallon cans that will give that amount of paint and have less than 5 gallons as a remainder.  The remainder will be taken from the 1-gallon sized cans.  You are then ready to figure the cost.