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