$(2)y=\begin{cases}{ 2.5 (15≤x\lt 30) }\ \\ { -\frac {1}{15}x+4.5 (30≤x≤45) } \end{cases}$
$(3)當(dāng)?shù)谝淮尉嚯x家2km時(shí),2÷\frac {1}{6}=12(min)$
$當(dāng)?shù)诙尉嚯x家2km時(shí),把y=2代入函數(shù)有 -\frac {1}{15}x+4.5=2,解得x=\frac {75}{2}$
$綜上,小明離家2km時(shí),離家時(shí)間為12min或\frac {75}{2}min.$
