Question

The temperature at 7pm was -5°F. The temperature began changing at the rate of -2.5°F per hour. What was the temperature at 10pm?

Answer 1

AT 10 PM the temperature is: -12.5°F

Explanation:

Notice that from 7PM to 10 PM, there are 3 hours difference.

At 7 PM the temperature was -5° F, then one hour later, (at 8PM it was -2.5°F less, that is: -7.5°F.

One hour later (9PM) : -7.5°F - 2.5°F = -10°F

Another hour later (10PM) : -10°F - 2.5°F = -12.5°F

The problem could also be solved in one step by multiplying the rate of change times "3" (due to the 3 hours elapsed between 7PM and 10PM), and using it to modify the starting value (-5°F):

At 10 PM: -5°F + 3 *(-2.5°F) = -5°F - 7.5°F = -12.5°F

Answer 2

-12.5 degrees

Explanation:

We start at -5 pm

from 7pm to 10 pm is 3 hours

it drops 2.5 degrees per hour

-2.5 *3 = -7.5 degrees

-5 + -7.5 = -12.5

The temperature at 10 pm is -12.5 degrees