A hen has two legs and a rabbit has four legs, so three hens have six legs and four chickens have sixteen legs.  $6+16=22$ 
Three hens and four rabbits are in a cage. How many legs do they have?~(adapted from $$2011$$ Math kangaroo Problems, Level $$1-2$$, Question \#$$4$$) 
a: $$12$$ 
b: $$15$$ 
c: $$18$$ 
d: $$22$$ 
e: $$24$$ 