Cagney can frost a cupcake every $$20$$ seconds and Lacey can frost a cupcake every $$30$$ seconds. Working together, how many cupcakes can they frost in $$5$$ minutes? ($$2012$$ AMC $$10\rm A$$ Problem, Question \#$$11$$) 
选项为:A:$$10$$  B:$$15$$  C:$$20$$  D:$$25$$  E:$$30$$  请回答其中一个选项.