Find the mean and median help! 
I feel like there isn't much information, but the answer for the mean is 79. The median is said to not be able to be found. Can someone explain this?
Thank you!

Regarding the mean, suppose that the sum of the 20 values is $S$, and the sum of the 30 values in the other group is $T$. Then you know that $\dfrac{S}{20}=85$ and $\dfrac{T}{30}=75$, which lets you find $S$ and $T$. The mean of both groups together is then
$$\dfrac{S+T}{50} = \dfrac{20\cdot85+30\cdot75}{50}$$
