1. The problem statement, all variables and given/known data For how long you have to let bottle in a river, if you want to cool a liquid inside from 22 °C to 12 °C, when river is 8 °C? 2. Relevant equations Q=cm(T1-T2), Qc=l*S(T-Triver)/d *t 3. The attempt at a solution I know how to solve it for a case when we substitute in equation Qc for conduction of heat T by average temperature of bottle from starting and finishing state. But I believe it should be possible to solve it as a differential equation for changing "T" in Qc. But I don't know how to find out dependency T(t), t- time. Or should I do it some other way? Thank you for your help and advice.