i think:
d-ducks
c-chickens
Ziyu has 240 more ducks than chickens in her farm:
d=c+240 or c=d-240
She sold 2/5 of the Ducks:
d-2/5d=3/5d
and bought another 450 chickens:
c+450
As a result the number of chickens is now twice the number of ducks:
c+450=(3/5d)*2
So let s put in term s of c-chickens c=d-240:
d-240+450=(3/5d)*2
d+210=6/5d
210=6/5d-d
210=1/5d
d=210*5
d=1050
Answer:1050