Niki usually leaves her cell phone on. If her cell phone is on but she is not actually using it, the battery will last for $$24$$ hours. If she is using it constantly, the battery will last for only $$3$$ hours. Since the last recharge, her phone has been on $$9$$ hours, and during that time she has used it for $$60$$ minutes. If she doesn\textquotesingle t use it any more but leaves the phone on, how many more hours will the battery last? 
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When not being used, the cell phone uses up $$\dfrac{1}{24}$$ of its battery per hour. When being used, the cell phone uses up $$\dfrac{1}{3}$$ of its battery per hour. Since Niki\textquotesingle s phone has been on for $$9$$ hours, of those $$8$$ simply on and~~being used to talk, $$8\left(\dfrac{1}{24}\right)+1\left(\dfrac{1}{3}\right)=\dfrac{2}{3}$$ of its battery has been used up. To drain the remaining $$\dfrac{1}{3}$$ the phone can last for $$\dfrac{\dfrac{1}{3}}{\dfrac{1}{24}}=\boxed{(\text{B})8}$$ more hours without being used. 