Problem: a certain elevator has a safe weight limit of 2000 pounds . what is the greatest possible number of people who can safely ride on the elevator at one time with the average ( arithmetic mean ) weight of half the riders being 200 pounds and the average weight of the others being 230 pounds ? 
Options: a ) 7 , b ) 8 , c ) 9 , d ) 10 , e ) 11
c