The general expression for the change in temperature between 8am and 10am
[tex]\delta_{8-10}= \delta_{8-9} + \delta_{9-10}[/tex] where [tex]\delta[/tex] stands for change in the subscripted time interval.
For city A:
[tex]\delta_{8-10} = 4 - 7 = -3\enspace ^\circ F[/tex]
For city B:
[tex]\delta_{8-10} = -6 - 2 = -8\enspace ^\circ F[/tex]
In city A, there is an overall drop of 3 degrees (negative value). In city B, the temp drops 8 degrees.
Among A and B, city B has a greater change in temperature (8 vs. 3)
