Answer :
[tex]v_1=40\ mph\\v_2=60\ mph\\t_2=t_1-8\\\\v=\frac{s}{t}\to s=vt\\\\s_1=s_2\\\\v_1t_1=v_2t_2\\\\v_1t_1=v_2(t_1-8)[/tex]
[tex]40t_1=60(t_1-8)\\\\40t_1=60t_1-480\\\\40t_1-60t_1=-480\\\\-20t_1=-480\ \ \ \ /:(-20)\\\\t_1=24\ (h)\\\\t_2=24-8=16\ (h)\\\\Answer:16h.[/tex]
[tex]40t_1=60(t_1-8)\\\\40t_1=60t_1-480\\\\40t_1-60t_1=-480\\\\-20t_1=-480\ \ \ \ /:(-20)\\\\t_1=24\ (h)\\\\t_2=24-8=16\ (h)\\\\Answer:16h.[/tex]