Sanya noticed that the temperature was falling at a steady rate of 1.4 degrees every hour after the time that she first checked her outdoor thermometer. By 6 a. m., the temperature had fallen 21 degrees. Which expression can you use to find how many hours earlier she had first checked the thermometer?

The expression to find how many hours earlier Sanya had first checked the thermometer is: .

Step-by-step explanation:

The rate at which the temperature was falling is, 1.4 degrees per hour.

Sanya checked the temperature at 6:00 AM and found out that the temperature had fallen 21 degrees.

Compute the number of hours it took for the temperature to fall to 21 degrees as follows:

It took 15 hours for the temperature to fall to 21 degrees.

Then 15 hours before 6:00 AM is, 3:00 PM.

Thus, the expression to find how many hours earlier Sanya had first checked the thermometer is: .