Answer:
[tex]S_a_v_e_r_a_g_e=48km/h[/tex]
Explanation:
Ok, the average speed can be calculate with the next equation:
[tex]S_a_v_e_r_a_g_e=\frac{Total\hspace{3}distance}{Total\hspace{3}time}[/tex] (1)
Basically the car cover the same distance "d" two times, but at different speeds, so:
[tex]Total\hspace{3}distance=2*d[/tex]
and the total time would be the time t1 required to go from A to B plus the time t2 required to go back from B to A:
[tex]Total\hspace{3}time=t1+t2[/tex]
From basic physics we know:
[tex]t=\frac{d}{S1}[/tex]
so:
[tex]t1=\frac{d}{S1}[/tex]
[tex]t2=\frac{d}{S2}[/tex]
Using the previous information in equation (1)
[tex]S_a_v_e_r_a_g_e=\frac{2*d}{\frac{d}{S1} +\frac{d}{S2} }=\frac{2*d}{\frac{d*S2+d*S1}{S1+S2} }[/tex]
Factoring:
[tex]S_a_v_e_r_a_g_e=\frac{2*S1*S2}{S1+S2}[/tex] (2)
Finally, replacing the data in (2)
[tex]S_a_v_e_r_a_g_e=\frac{2*40*60}{60+40} =48km/h[/tex]
