it dropped from 0 to -20
-20 -0 = -20
divide by the time of 5 hours
-20/5
-4 degrees per hour
average change per hour=(total change)/(time elapsed in hours)
total change is rom 0 to -20 or -20
hours=5
o average change is [tex]\frac{-20^oC}{5hr}=-4 \frac{^oC}{hr}[/tex]