Answer:
[tex]\bar v = 60\,mph[/tex]
Step-by-step explanation:
The distance travelled by Lisa in the first four hours of her trip is:
[tex]\Delta s = \left(50\,mph)\cdot (4\,h)[/tex]
[tex]\Delta s = 200\,mi[/tex]
The distance remaining and her average speed are, respectively:
[tex]\Delta s_{R} = 440\,mi - 200\,mi[/tex]
[tex]\Delta s_{R} = 240\,mi[/tex]
[tex]\bar v = \frac{240\,mi}{4\,h}[/tex]
[tex]\bar v = 60\,mph[/tex]
