Suppose there are $$x$$ ducks in the yard. Since there is an equal number of pigs, ducks and chickens, $$2x+4x+2x=144$$, $$x=18$$. 
In the yard there is an equal number of pigs, ducks and chickens. Together, they have $$144$$ legs. How many ducks are there in the yard? 
A) $$18$$ 
B) $$21$$ 
C) $$35$$ 
D) $$42$$ 
E) $$43$$ 