James can frost a cookie every $$12$$ seconds and Lacey can frost a cookie every $$15$$ seconds. Working together, how many cookies can they frost in $$10$$ minutes? (Adapted from $$2012$$ AMC $$10\rm A$$ Problem, Question \#$$11$$) 
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In $$600$$ seconds ($$10$$ minutes), James will frost $$\dfrac{600}{12}=50$$ cookies, and Lacey will frost $$\dfrac{600}{15}=40$$ cookies. Therefore, working together they will frost $$50+40=\boxed{(\text{D})90}$$ cookies. 