Average speed and RMS speed, given speeds of objects

I am given the speeds of eight objects, and I have to find the average speed and the RMS speed of these objects.
I tried taking the actual average of these speeds: $frac{1}{8}Sigma(v_i)$, but I get $7.4$ $m/s$ instead of $7.5$.

I know that $v_{average}=2 sqrt{frac{2 k_B T}{pi m}}$ and $v_{rms}=sqrt{frac{3 k_B T}{m}}$, but I do not see how these formulas could help me in any way as I do not know the mass of the given objects, just their velocities and their number.

I also know that $f(v)=(frac{m}{2 pi k_B T})^{frac{3}{2}} 4pi v^2e^{frac{-mv^2}{2k_BT}}$ is the probability per unit speed. I feel like it might have some relevance in terms of weighting in both cases but again, I do not know the masses of the objects. I tried assuming they all had the same mass and did not get the result I wanted.

I tried to find the rms speed by taking $sqrt{(Sigma (vi-v_{average})^2)}$ but I get 11.2 instead of 8.5.

Any suggestions on the right direction?

NOTE: The objects’ velocities in m/s are 2,4,5,5,8,9,11,15 (a user asked previously. I can remove if too specific).