trick question - you have birds, ducks and chickens in the statement part of the question yet you only want the number of birds in the answer.

You said 10 birds cost 1 riyal but you want to know how you can have a mix of 100 birds for 100 Riyal. It's not possible because 100 birds would cost 10 riyal

~I know nothing!~
Sergeant Schults from Hogan's Heros