Yes. You do know the median of the 20 set and the 50 set but is impossible to determine the median of these two combined without knowing the start and ending point
In the stem the told you which is the median and this is ok. It is data the stem provided. Ok so far so good. I take for granted.
But if I have to combine the two data sets and then see which is the median is impossible to assess.
Where I start and when I will end....who knows
I could start from 1 to 50 or from 127 to 177 the median is quite different
Now is more clear ??