A dosage rate of $250$ mg/hr is reduced by $5$ mg/hour each hour.

1. Complete the table for a dosage rate reduced by $5$ mg/hour each hour.

   t (hours)	Dosage (mg/hr)
   $0$
   $1$
   $2$
   $3$
   $4$

What is the rate of change $m$ between any two ordered pairs in the table? $m=$