Unclear explanation in "Some Median Problems" Video

GRE GRE Quant Problem Solving
1

I’m unclear on the explanation provided (or lack thereof) for some of the problems in the “Some Median Problems” Prepswift lesson - Some Median Problems - PrepSwift

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  1. How did we conclude that the sequence is a symmetrical set, for all three cases?
  2. How is the median 180 for all three cases?

Sometimes the explanations provided are very vague.

2

  1. In every series be it of multiple of 4 or 5 or 6 the terms have equal distance between each other. For example, multiples of 5 - 120,125,130,135….240.

  2. Both 120 and 240 are the first and last multiples of the nos. 4,5 and 6.
    So, (first multiple + last multiple)/2 would give us the median of the all 3 series, which is 180. as a+b+c is asked it would be 180*3 = 540.