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?

Negative 21 divided by negative 1.4
Negative 1.4 divided by negative 21
Negative 21 divided by 1.4
21 divided by negative 1.4

1 answer

To determine how many hours earlier Sanya had first checked the thermometer, we can set up the situation mathematically.

If the temperature fell 21 degrees total and the rate of decrease was 1.4 degrees per hour, the number of hours (let's call it \( h \)) can be found using the relationship:

\[ \text{Total change in temperature} = \text{Rate of change} \times \text{Time} \]

In this case:

\[ 21 = 1.4 \times h \]

However, we need to express this in terms of finding \( h \) from the given data:

To isolate \( h \), we rearrange the equation:

\[ h = \frac{21}{1.4} \]

This tells us that we need to divide the total change in temperature (21 degrees) by the rate at which it is falling (1.4 degrees per hour).

Among the options provided, the correct expression to find how many hours earlier she had checked the thermometer is:

\[ \frac{21}{1.4} \]

Since it’s expressed in terms of how much the temperature changed positively (21 degrees). Therefore, the correct choice from your list would be:

21 divided by 1.4