My aunt can fold $$4$$ paper cranes in $$1$$ minute. My uncle can fold 3 paper cranes in $$1$$ minute. Together they fold $$7$$ paper cranes in $$1$$ minute. It takes them $$42\div7 = 6$$ minutes to fold $$42$$ paper cranes. 
My aunt can fold $$16$$ paper cranes in $$4$$ minutes. My uncle can fold $$15$$ paper cranes in $$5$$ minutes. How long would it take them to fold $$42$$ cranes if they work together at those rates?　 
  * $$6$$ minutes 
  * $$9$$ minutes 
  * $$12$$ minutes 
  * $$13$$ minutes 