Could anyone give me a simple word problem that boils down to a differential equation to solve?

Question

jamesj · Answer

The cup of tea in from of me is currently at 70 C and the air around it is 20 C.  The tea cools at a rate proportional to the difference between its temperature and the air temperature.  When will the tea be at 50 C and I can drink it?

anonymous · Answer

i am going to bet that we can do this without any differential equation, yes?

anonymous · Answer

oh actually no, because i have no idea what the rate of decrease is

anonymous · Answer

am i confused (probably) or don't we need one additional piece of information?  suppose the tea was tar?

anonymous · Answer

|dw:1326471386752:dw|$$\frac{dT_t}{dt}=-k(T_t-T_a)$$this is what i have so far. brb