[tex]\dfrac{s_1+\cdots+s_8}{8}=Q\implies s_1+\cdots+s_8=8Q[/tex]
[tex]\dfrac{s_1+s_2+s_3}{3}=Z\implies s_1+s_2+s_3=3Z[/tex]
Substituting into the first equation, this means
[tex]s_4+\cdots+s_8=8Q-3Z[/tex]
Divide by 5 to get the average:
[tex]\dfrac{s_4+\cdots+s_8}{5}=\dfrac{8Q-3Z}5[/tex]
