Suppose that a cup of boiling hot coﬀee (ie its temperature is $100^oC$) is left in a $20^oC$
room and sits until it has cooled to $60^oC$. The cup of coﬀee is then taken outside where
the temperature is $5^oC$. After 15 minutes, the coﬀee is now $25^oC$. How long has the
coﬀee been cooling in total?

Normally I would have some sort of idea where to start, but with this I have absolutely no idea how. Can someone give me some guidance on this?

Thanks.