We can set this problem up as an algebraic equation, which we can then solve to find our answer.
We start at -3. So we can start our equation with that:
However, this doesn't represent any change in temperature, just that we start at -3. To show change, we will use the variable , where
stands for the number of hours. Each hour accounts for a change of -2.5 in degrees, meaning that we must multiply our
by -2.5. We can now add this term into our equation:
We are trying to find when we will reach -18 degrees, so we will set our entire equation equal to -18 and solve:
It will take us 6 hours, or choice A.