First let's calculate the difference in temperature:
[tex]78-85.5=-7.5[/tex]The change is -7.5°F. If the rate of change is -1°F every 18 minutes, we can use a rule of three to find the time needed for -7.5°F:
[tex]\begin{gathered} -1\degree F\to18\text{ minutes} \\ -7.5\degree F\to x\text{ minutes} \\ \\ \frac{-1}{-7.5}=\frac{18}{x} \\ x=18\cdot7.5 \\ x=135 \end{gathered}[/tex]So the time needed is 135 minutes. In hours, that is equal to:
[tex]\frac{135}{60}=2.25[/tex]So we have a time of 2.25 hours.
