If $$2$$ hoots $$=1$$ holler, then $$(10\times 1)$$ hollers $$=(10\times 2)$$ hoots. 
If $$6$$ hoots $$=3$$ hollers, then $$10$$ hollers $$=$$~\uline{~~~~~~~~~~}~hoots. 

a: $$5$$ 
b: $$13$$ 
c: $$18$$ 
d: $$20$$ 