jmc

Mr. Fat eats cereal at a rate of $\frac{1}{20}$ pound a minute, and Mr. Thin eats cereal at a rate of $\frac{1}{30}$ pound a minute. Together, they eat cereal at a rate of $\frac{1}{20}+\frac{1}{30}=\frac{1}{12}$ pound a minute. At this rate, it will take them $\frac{3}{\frac{1}{12}}=\boxed{36}$ minutes to eat 3 pounds of cereal.