Whenever you are calculating a rate of change, you need to determine what the numerator and denominator will be in the ratio. In most cases, time will be in the denominator if it is one of the variables.

In this case, you are looking for a ratio of (Change of temperature)/(Change of Time). Change is always figured by subtracting the initial from the final. These facts produce the following equation.

(Final temperature - initial temperature)/(Final time - initial time)

(61 - 85)/(7 - 1)

(-24)/(6)

-4 degrees per hour

The temperatured dropped an average of 4 degrees per hour.