Speed of Grace in morning :
[tex]v_m = \dfrac{2\dfrac{5}{8}}{\dfrac{1}{3}}\\\\\\v_m = \dfrac{\dfrac{21}{8}}{\dfrac{1}{3}}\\\\v_m = 7.875\ mph[/tex]
Speed in evening :
[tex]v_e = \dfrac{\dfrac{21}{8}-\dfrac{5}{4}}{\dfrac{1}{6}}\\\\v_e = 8.25\ mph[/tex]
Average speed :
[tex]v = \dfrac{v_m + v_e}{2}\\\\v = \dfrac{7.875+8.25}{2}\\\\v = 8.0625\ mph[/tex]
Hence, this is the required solution.