On a farm, the number of chickens was more than the number of ducks. After 20% of the chickens were sold, there were 800 less chickens than ducks on the farm. How many ducks were there on the farm? ___ducks
so from (2)
multiply through by 5 to get whole numbers
substitute for c in (1).
but c must be a whole number so
in eqn (3) the term
Combining both of these conditions gives: