I have the following values $\overline{b}$, and $\overline{u}$
which is : $\overline{b} = \dfrac{b_1+b_2+b_3+b_4}{7}$, $\overline{u} = \dfrac{u_1+u_2+u_3+u_4}{7}$
is there any way I can get the following if I have only value of $\overline{b}, \overline{u}$?
$$\frac{b_1/u_1 + b_2/u_2 + b_3/u_3 + b_4/u_4}{7}$$
Basically I want to convert average of sums into sum of averages
It cannot be done. The average of the $\frac{b_i}{u_i}$ cannot be recovered from the average of the $b_i$ and the average of the $u_i$.
For suppose that $b_1=b_2=b_3=b_4=6$ and $u_1=u_2=u_3=u_4=3$. Then
$$\frac{b_1}{u_1}+\frac{b_2}{u_2}+\frac{b_3}{u_3}+\frac{b_4}{u_4}=8\tag{1}.$$
Suppose now that $b_1=b_2=b_3=b_4=6$, and $u_1=u_2=2$ and $u_3=u_4=4$. Then
$$\frac{b_1}{u_1}+\frac{b_2}{u_2}+\frac{b_3}{u_3}+\frac{b_4}{u_4}=9\tag{2}.$$
Note that in the two examples, the $b$'s were the same, and the $u$'s had the same sum, and therefore the same average.
But the sums of the $\frac{b_i}{u_i}$, and therefore their averages, are not the same.