In $$300$$ seconds ($$5$$ minutes), Cagney will frost $$\dfrac{300}{30}=10$$ cupcakes, and Lacey will frost $$\dfrac{300}{60}=5$$ cupcakes. Therefore, working together they will frost $$10+5=\boxed{(\text{B})15}$$ cupcakes. 
Cagney can frost a cupcake every $$30$$ seconds and Lacey can frost a cupcake every $$60$$ seconds. Working together, how many cupcakes can they frost in $$5$$ minutes? (Adapted from $$2012$$ AMC $$10\rm A$$ Problem, Question \#$$11$$) 
1: $$10$$ 
2: $$15$$ 
3: $$20$$ 
4: $$25$$ 
5: $$30$$ 