Math

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